In general, to affect T:Group, only one member of the group must be in range. So if going with T:Group, R:Arc, one needs an arcane connection to a single member of the targeted group. However, "the components of the group must be close together in space, and the group itself must be separated from any other things of the same type" (ArM5, p.113).
It then seems to me that if you hold five arcane connections, each to one of five grogs that are not close together in space, you can't affect those five grogs with a single T:Group, R:Arc spell. The argument that the five grogs should be a group just because in some sense their arcane connections form a group appears invalid to me.
Question 1: Am I right?
Question 2: What is the most effective way to target those five grogs - is it really to just target them one by one?
I would probably be lenient here and say that if you have a group of fixed arcane connections, they can easily be separated from other targets for arcane connection range, regardless of where the targets are.
But can you make use of more than one Arcane Connection at once?
Perhaps one could cast a spell at the group of Arcane Connections (with Range: Touch or Voice), and have it redirect through the ACs to the targets they are ACs to, but I am not at all sure that is even possible with Hermetic Magic as it is presented - i.e. it might require a breakthrough to make it possible.
Right. There is no way around "close together in space" for T: Group. Puttiing the grogs in uniform or such might provide the "separated from any other things of the same type", if they happen to be in a melee.
Mastering a T: Ind spell for (ArM5 p.87) Multiple Casting would work well, if you have five ACs to five grogs. Maybe start with fewer grogs, and build up the Mastery over time.
Hmm. For once I agree with OneShot
"Simultaneous uses of an effect" in the same item should not be allowed -- I think they go against the rules, certainly in spirit, probably in letter ("You can use one effect from one item each round", ArM5 p.100).
Because of that, I assume that the effect in Bell of Summoning is ambiguously worded, and one has to keep tolling it for 6 rounds so as to "reach" its 6 intended targets, one after the other. Note that this interpretation does not really hinder its intended function.
Oh, good catch. The horn targets a Group of magi by having an Arcane Connection to each magus; and it seems that the intent of the author (Andrew Gronosky -- on the forum by any chance?) was indeed to ensure that every magus is affected no matter where they stand relative to each other.
That said, in principle the general rule of targets needing to be close together is never explicitly contradicted, so one could read the effect as failing if the "target group" is not ... well a Group according to ArM5 p.113. Then again, if the effects only worked on such a Group, there would be no need to require a connection to each member. In fact, the entire premise of CrIm at R:Arc appears fishy: the Target of a Creo Imaginem sound effect should be the intended sound (to which no Arcane Connection exists before its creation) not the person meant to hear it.
To be honest, it seems to me that the Horn of the Champions is intended to be heard by, and subsequently summon, multiple magi wherever they may stand relative to each other, and that the author handwaved the rules somewhat. This might be due for the errata ... if there's anyone still taking errata for Ars Magica?
Actually, the T: Group summoning effect of HoH:S p.13 box The Horn of the Champions is off the rules only for a cosmetic effect.
Designing it for T: Ind instead of T: Group frees just the 10 levels for (ArM5 p.98 box) Unlimited uses. By blowing the horn once for each champion, the castellan can then summon all of them without rules problems.
Replacing T: Group with Unlimited uses in an item might also be a trivial solution for other such problems with multiple ACs to scattered individuals.
If there were a clear rule, uniformly applied throughout the line, that I did not like, I'd definitely take your approach and "house rule it away".
But here the problem is to understand what the rule says, based on its wording, and how it was applied throughout the line -- keeping in mind that often authors err too, and that indeed the majority of errata is about fixing some specific example of a character, spell or device so as to bring it in line with a specific rule it violates.
Actually, it seems to me the Horn could be the "source" of a cool Minor Hermetic Virtue. Let's say...
Master of Arcane Connections
You can do more with Arcane Connections than most magi.
First of all, your magic can create things in touch with anyone or anything it has an Arcane Connection to. For example, you can invent and cast a sixth magnitude version of Pilum of Fire that makes a target you have an arcane connection to burst into flame. This is usually impossible without a Wizard's Tunnel or similar conduit, because created objects are the Targets of the effect creating them, and there is no Arcane Connection to them before their creation.
Second, if you hold Arcane Connections to multiple targets, you can treat that set of targets as a Group, and affect it with T:Group R:Arc spells -- irrespective of whether the individual members are close to each other or separated from similar targets.
Third, you can fix Arcane connections more efficiently. You can fix multiple Arcane Connections in a single season of Lab work, one for each point of Magic Theory. Alternatively, you can fix a single Arcane Connection over a single year as a side project that only counts as a seven-day seasonal distraction from your other activities; you must have access to your laboratory for the whole time.
Hmm. Actually, not for complexity, but it might actually be that "Arcane Group" is a "Special" Target that -- since it violates no limits of magic -- can be used with non-Spontaneous magic at a slightly higher cost than the closest existing Target (i.e. Group), ArM5 p.114.
Your second point is the way that my group views Arcane Connections, in that for spells/effects designed to affect a Group with Range AC they form a Symbolic Group. It also allows the spells/effects to function as written in varies books.
For play groups that do not like that interpretation, fitting it into something like a new Virtue seems to be an effective way forward.
You could also make it so that people with the Virtue can design spells and effects to take advantage of it, which others could learn and cast or enchant from a lab text. However without the Virtue they would not be able to design a spell or effect different from what was available in the lab text.
Other possible ideas would be things like expanding how long an AC is valid for them (such as double normal), increase the speed with which they can fix an AC, or allow them to fix multiple AC in the same season (if you do not want to reduce the time, say a number equal to MT or some fraction of MT).
Heck, you could create a whole small group of Virtues centered around it that interlock and work together. Would make a very interesting area of focused research for a Bonisagus.
If your play group does not allow this by RAW, then it would be a fairly straight forward Minor Breakthrough for a custom Target.