Really it is simple- if it is direct damage it sets the people on fire, essentially like a shotgun where every single round hits it's own target (visually, that is extremely impressive), and anything not part of that group (say the horses they are riding, or if you target horses the riders...) will not be affected.
if it is indirect damage it requires a finesse roll, but can take out things which are not necessarily part of that group.
One thing I don't understand from the current tread is this. Why the empire is angry with Vulcan, enough to send 500 men against him?
Was he selling enchanted items to its rivals or enemies? If so, how did the empire learn that Vulcan was the source of those items?
Just curious...
I think the main thing here is that Paabo's interpretations seem to work for him and, presumably, his group, so they should definitely stick to them.
As for my own tastes - I would consider the actions of Artorias to have been either legal or not depending on circumstances, but also consider 500 soldiers to not be a serious threat to a combat-able covenant.
Vulcan's actions have endangered his sodales, and in that he is guilty of an Hermetic crime. The proper response to this isn't to be decided on by a Hoplite, however, but rather by a Quaesitor or, better yet, a Tribunal. In my opinion the Quaesitors the characters consulted should have insisted on convening an emergency Tribunal. Failing that, a Quaesitor would have passed a field judgment but only after personally investigating the case. In either case I think the act of selling items to mundanes through intermediaries is perfectly legal, as is refusing to sell to them, and to construe this as violating the code will not fly in Tribunal, especially one with Verditius and other trade-oriented magi around. Supposing the problem is that the items are unacceptably strong, this would probably constitute a Low Crime.
My solution would be for Vulcan to notify the Empire he's stopping to make the items in a way that would make the Empire pursue him as in your scenario. Should the Empire indeed try to catch him, he should warn the Empire that he'll die fighting and that his items will fail upon his death. When the Empire will try to capture him by force, arrange for him to "die" to mundane appearances in the battle. Then arrange for all his items to fail, through PeVi spells, agents, or so on (through specialists paid for by Vulcan, as his punishment for bringing up the whole mess; the covenant can take care of some of them, though, for an extra adventure). Vulcan would furthermore be disallowed to sell any magic items to mundanes (or to anyone?) or let any mundane see him (without disguise/illusion) for 70 years.
True, the 'target' in pilum of fire, BOAF or even Crystal Dart are all the actual source of the damage, not the person hit. Which means you'll end up with 100 POFs hitting the first unlucky bugger 
Admittedly, as a show of shock and awe....
Or a somewhat easier interpretation is to just have the group of recipients conform to the T:group requirements of being discrete and separate which can be perceived by the caster.
Make sure your size +1 since group == 10 individuals.
It would seem to me that 100 pillum of fire could be 'fanned out' in an arc reaching about 50 paces wide with 2 pillum pre pace distance, allowing each person to be essentially 'targeted' by 2 pillum. That wouldn't so much be targeting as sweeping an area...
of course 10 pilum which is each 10x the normal size could be interesting as well... not sure exactly how that would work...
Yeah I think we've been playing assuming that a 'group' fireball is just a very large fireball, doing 30 damage to anyone caught in the explosion- the explosion being roughly the size of 10 individuals standing near each other. Reasoning: creo spells work a little differently, with the target being whatever is created. The way I interpret this is that a group fireball is a fireball the size of a group- not an exact measurement for sure, but I think we can all imagine about how big that would be. The book says that you can't have a creo spell with target: room fill a room with something, you would need a group+1 spell to do so. I think this supports my idea that my CrIg spells are filling a volume of space, not discretely targeting 10 (or however many) men.
As for Vulcan: he was basically making cell phones and selling them to the Empire. The Empire was using the phones to control their troops, thus giving them a massive advantage on the field over every other army. I'm not clear as to the movements of the Holy Roman Empire in this setting, but I imagine this allowed them to consolidate power fairly easily in areas in which they previously could not. When Vulcan found out he was being investigated, he decided to stop.. but his contact in the Empire didn't like that idea. He gave him a choice: either continue making the items or be crushed. He chose the second, and the Empire made good on their threat. I think if Vulcan had just continued to make the items until Tribunal, he would have been mildly punished and told to stop, perhaps receiving aid from the order to do so. Instead he brought an army to our door and almost got all of us killed. His chickens came home to roost, and instead of standing with him to fight we did the most logical thing in regards to our own survival. Meta-game, this was probably a mistake. In-game, this is the best plan our young magi could come up with, and we'll stand by it.
And now your covenant has interfered with mundanes, thereby bringing ruin upon your sodales. 
T:Group doesn't create a large fireball, because it's much more flexible than that. If you're using it as a large fireball, I suggest it be redesigned to be T:Ind, Size +3 which would be creating a fire capable of doing +30 damage that is 100 times larger than the Ball of Abysmal Fire. This creates a spell that can't be used in small spaces, unlike the Group version, which could theoretically target select people if they are a discrete group and your troupe agrees that T:Group for Creo spells applies to the recipients of the spells, rather than the items created. Creo and targets get a bit wonky, it just needs to be resolved and dealt with consistently.
This is an example of where the troupe needs to come together and decide how something really works, because a lot of people could have distinctly different ideas on how it does work. Your character wouldn't be nearly as formidable in sagas that I've run or play in because how we thing T:Group works.
It seems to me the rules are pretty straightforward in this regard. I'm creating an individual fireball roughly 10x the size of a large campfire, not creating personal fireballs for each target in the area.
IMO the spell to hit large groups with, which might bypass these hurdles would be PeCo. The target is then the people, not anything created. You'd still have to target a grouping of people that are to the caster one discrete group. To do that you'd likely want to make it Range Sight, Target Group +X. Where X is a multiplier large enough to handle say 1000 men (so +2 magnitudes). From there it's a question of having the right field of vision and how much damage you want to cause. By this point you're likely in the Ritual range for the level (55+), especially if you want to inflict a Heavy Wound or higher (which would almost certainly take care of the army, though many might live if wasn't an Incapacitating wound or Death).
The option then is not to use a Ritual but rather a magic item, to give you the versatility. Use a version of Wizard's Sidestep to leave an image of yourself on the ground that talks with and distracts the army, then while floating above (invisibly) you can see the whole of the army and lay waste to it. Now, regardless of your intentions this amount of carnage would likely be considered to bring ruin upon your sodales. But the immediate problem (the army) is taken care of.
Maybe PeCo can fix all the problems in the world, but my PeCo total is 21 plus a die, while my CrIg total is effectively 54 plus a die. Fire is always the answer; there has never been a problem that couldn't be solved with more fire 
Then there's your answer. CrIg targets are the fire itself. Rather than creating a bunch of small intense fires, make 1 individual intense fire, target Ind and then keep adding size magnitudes to increase it in size and dimensions. Drop it on top of the group in front of you (maybe a bit behind the front rows) and burn them up, since the target is the fire, you need to see the fire, not the men you're burning. Cast it forcelessly so that you and anyone with you with any MR is protected and that should do the trick (assuming it's big enough to reach you from where you cast it and where you stand). It's not all that accurate, but a big fire doesn't need to be accurate. The ones underneath it are barbecue, the ones behind it are going to flee and you sidestep all of the targetting issues. Casting it forcelessly does leave one problem; some of the soldiers might manage to sneak by with a smidgen of MR provided by the Divine from Relics and the like. But that would be the odd person; unless they were accompanied by a priest or other some such holy person, then your problems have upgraded dramatically.
Are they? Why not just make a spell that is T:Ind, Size +2, which is equivalent to a T:Group, Size +1. In one case, you have created Ignem capable of doing +30 damage which is 100 times the size of a Ball of Abysmal Fire, and in your case, you're suggesting that T:Group gives you the flexibility to create a fireball 100 times the size?
The apparent goal of the spell, as described by you, is to deliver a fireball big enough to injure about 100 people. How big is that, then? The size of the fire delivered by Ball of Abysmal Flame is probably pretty small, since it affects only one person. Is it baseball, basketball or medicine ball sized? Basketball is probably the right size. So T:Group would create 10, Size +1 to group would create 100 of these. If it creates one, 100 times the size, does that mean it affects 100 people? Or does it mean it affects a volume 100 times greater than the base size of Ball of Abysmal Flame?
I'm going to go with basketball sized. Which is a 9 inch diameter, and has a volume of 4/3pir^3 (4/33.141594.5^3)=~382 cubic inches. Size +2 on that is 38,200 cubic inches, which is a diameter of ~42 inches... Size +3 is 100 times bigger than the base size in volume, or 382,000 cubic inches, with a diameter of 90 inches, we're talking now, but that's still not big enough to affect 100 people...
I'm affecting a space the size of a group+1. The fireball coming out of my hand is purely cosmetic. Remove the idea of the fireball in your head and just imagine that I want to set fire to a space roughly the size of 100 men. Does that make it easier? There is of course an explosion when the fireball hits, and the fire moves outwards from where it hits to fill the space of 100 men- these are all just cosmetic additions. When it comes down to it we're deciding how many squares on a piece of graph paper I'm affecting, which is easily definable by the rules I quoted.
I'll put it this way: a fireball aimed towards an individual is really placing an individual-sized fire in one square momentarily. To do the same with a group would be to create 10 fires in 10 adjacent squares, while ind+1 would create 1 fire occupying 10 squares. Group+1 would be 100 fires in 100 adjacent squares, while ind+2 would be 1 fire occupying 100 squares. They're both accomplishing the same thing. You're getting hung up on the aesthetics of it, I think.
Also- who throws a fireball the size of a basketball?? This ain't no magic missile, this is a ball of abysmal flame. I'm whippin' boulders, not pebbles.
Above: MSPaint masterpiece
But that's not how it works. You can't affect a space of Group +1. What is a Group of Fireballs space? 10 individual fireballs? The fireball may be cosmetic, but the effect is not. You're wanting to create, in essence a fireball the standard size to affect 100 individuals. And I'm perfectly fine with that, but you have to apply the group rules consistently, and it only affects a discrete group, and you have to perceive it so. T:Group creates 10 fireballs, Size +1 applied to the group creates 100 fireballs. T:Individual, Size +3 creates 1 fireball of 100x the base size of whatever Ball of Abysmal Flame is. It could injure many more people in the vicinity, I have no problem with that, but that number isn't approaching 100, due to the size. Pick one and go with it, IMO.
If you apply it that way, then you need to perceive the Group targeted as being a group, IMO. I'm not hung up on the aesthetics, because, as demonstrated above, T:Group, Size +1 on the Group and T:Ind, Size +3 are completely different things.
No, that's not how it works. Size modifiers on individuals affect the base size of the form, which has nothing to do with the overall area affected. It's not aesthetics, it's math. 
T:Ind, Size +1 on a Pilum of Fire or Ball of Abysmal Flame is just a bigger flame, 10 times bigger than what would affect an individual, but that doesn't mean it affects up to 10 individuals. T:Group, I don't have much of a problem with, so long as you can perceive the group so affected, and yes, if you can perceive the group of 10, 100, 1000, you can affect them with appropriately sized spells. I have no problem with that. Where this seems to be is that you believe you can affect any 100 individuals, because of the size of your spell. I believe that's not consistent with the RAW.
Regarding the size of the spell and your picture, I view BoAFas a white hot palm sized ball of flame that burns holes through people. And Pila is, of course, a javelin sized spear of flame that is orange in appearance. If you create fireballs that big, you must have collateral damage from the BoAF, which it plainly, explicitly does not.
Ignore the visual for now. The target is group which equals 10 individuals. By then adding size +1 it becomes 100 individuals. The spell does the same damage whether cast at 1 person or 100 people.
JLinks, you're mistaken. The rules for Creo effects are different. I'm not affecting 100 individuals, I'm affecting the fire that I'm creating to make it the size of roughly 100 individuals. It doesn't matter how many people are standing in that area, I'm still making a fire of that size.
While a tiny ball of fire tossed from your hand is one way to visualize it, Artorias is all about being flashy. Yes, I agree a spirit ball-sized fireball is over-the-top, I was joking. But his fireballs would definitely be massive, (certainly larger than a basketball) and definitely be scary. Discretion is not his strong suit.
I'm aware that the rules for Creo are "different", I even stated as such much earlier in the thread. I even suggested that if you want to affect 100 individuals you would have to, in effect, HR that the rules for a Group must still hold for your BoAFs so delivered. Meaning you perceive the group as being discrete from others nearby of the same type. I have no problem with this as a HR, it is very reasonable.
Right you are about the 1 thing the size of 100 individuals, but it is of the base form, not people. Then you're not affecting 100 people. See above 100 individuals of either basketball sized or smaller. Probably not bigger. We know the size isn't 100 large camp fires or hearth fires. We know it has to be smaller and hotter than that. You probably could relatively easily create a Size +2 T:Ind fire that would engulf 100 people, but it's probably the size of 100 large campfires (maybe less) and it would do only +10 damage, or somewhere around +10. It's unwritten in the RAW, but seems implicit in the execution that there is an inverse relationship between the size of the individual of fire and the amount of damage it does.
Probably not, as the base size for Ball of Abysmal flame has to be small enough that it can't reasonably injure another, as that's explicit in the text. You can be flashy as you want, and have a cosmetic effect that is the size of a large camp fire, but the only part that does any damage is the small baseball, basketball, piece big enough only to hurt one human with +30 damage.
Edited many times after the fact, because I don't read what I post until I click submit. 