In a single gaming session, do people play a main character plus a grog, or do some people play their main char and others play a grog, or is this up to the troupe? I'd thought the former was standard, the storyguide's been doing the latter.
With us it varies... It's usually known who take part in particular story, and if these characters have a set player (like one of our main characters or favorite grogs) then that player will play that character. Othervise we just distribute the characters (grogs) on the depending on how many each player feels comfortable playing.
I've normally done the later in my games.
A typical sesion (to the extent that there is such a thing) starts out with the magi in council then the focus switches to an "away team" where each player plays only a single character.
We almost always play our magi, with maybe one or two of us also playing accompanying companions and maybe a shield grog. That sessions SG usually plays all the other grogs.
It depends on the situation.
I try to limit one character to player, which means that if players A and B are taking their magi, and player B normally plays player A's grog, the grog will go to player C. But it doesn't always work that way, and we have on more than one occasion had people play multiple characters in a session. Often that means that one player will take a few grogs, a developed shield grog and then a couple fodder-- I mean, other grogs, but sometimes it's just a complete mess.
Of course, its worse in covenant centered games where all the characters are around and have a reason to interact with the story.
Ditto on Master Tyrell's comments.
It's almost a formula in our game. We start in council, which always lasts way to long. Then we have an away team go on the adventure. Typically that away team consists of a mix of characters, Magi, companions, and grogs. We try to keep the magi to one or two. Any more and we get into "Jedi" problems. Tradition in our game, has it that Custos and Grogs do the brunt of the work in an adventure, with a magus throwing in spells for color and the cool effect. But mainly the companions do a lot.
An unofficial rule in our troupe is that we have to 'mix it up'. So if player A has a mage on the adventure. Player A can not play his own magus shield grog. Someone else must play the shield grog. This 'forces' the players to interact with each other rather than group up into mini-troupes that they control. That is Player A can't play Mage and Mage's shield grog and run off to their own thing. At least two players are going to be stuck together and that helps keeps thing from being player indulgences.
A variation of this rule is that no player can play the apprentice of their mage. This avoids the like minded or 'yes sir' senario of apprentices always doing exactly what their Master wants. When another player is involved the Apprentice-Magus relationship is always more complicated.
I only have two players in my troupe, and each plays a magus (the main character), as well as a companion to the other magi. Honestly, one player is more invested in his companion than the mage, because he spent the time to create a more interesting background and goals for the companion.
I try to keep the adventures focussed on the affairs of the mages. If a story only requires one mage to leave the covenant, then the other player plays the companions.
However, in all game sessions, I make sure to at least "check in" on all PC's, just to see what their up to.
Hope this helps,
eric
We have a troupe of seven, including the storyguide, and have found that adventures are best when we take one, no more than two magi on adventures-- depending on anticipated difficulty, but it's been a long time since more than one magus went abroad for an extended adventure.
The rest of us play the grogs that the magus wants brought along with him-- sometimes a Companion, but that also depends. It's not uncommon for us to each play one or two grogs, shifting characters as needed.
-Ben.
Of course it's up to the troupe. My troupe takes your SG's approach, and I think that I would definitely prefer it.
In my troupe, we usually play one character at a time. Sometimes people will play a main character plus one or more grogs; sometimes, someone will just play one or more grogs. Usually, if you're playing a main character plus grog(s), the grog(s) have a special relation to the main character, such as being that character's servant or bodyguard. We usually only do this with the more docile and thinly-designed of our grogs; the more richly thought-out grogs tend to get a dedicated player if they show up, and we try, within reason, to make it the same player (the grog's designer) all the time where that's possible.
The game I play in, we usually play one main character plus one (or occasionally 2) grogs. Mages are played more often than not, but that may be the stories we've been having. Guest players get grogs!
In my own saga so far most people only have a mage so they tend to play them if they turn up to the session.
I can recommend playing a story or two using only grogs... My troupe had a wonderfull session playing the covenant household during a visit from important guests. 
Also in stories where a single player plays a magus and the others are grogs or companions I find that too much of the responsibility for that story is put on the magus character. The story get assymetrical in a metagame sense because a single player (not character) has to make all the tough decisions.
In my experience each player usualy plays his one or two main characters if they are present (magus or companion). Those without main characters present play a single grog. More grogs are controlled by the SG or the players, but don't really get played - their actions are only briefly described and skimmed over.
Usually. There are no rules, just flow with it.
Yair
I'd like to second the comment on grog-only stories as being delightfully entertaining.
Two weeks ago, our troupe assembled for our weekly game, a continuation of a magus's exploration into a distant realm. The player playing the magus called at the very last minute and cancelled, leaving us high and dry as to what to do. On the spur of the moment, one player suggested that they all make grogs and I run a story about these new characters travelling to and joining the covenant. We proceeded with the plan and it was great fun.
As a side note, since I didn't want to spend a bunch of time making grog characters, I muscled them through the process in ten minutes. I told them that they were going to roll their characters' Characteristics instead of buying them, and then ingore any Characteristic-aimed Virtue or Flaw. This was new for them, none of them having played Ars before 5th Edition. The end result was that they loved it, and asked if, in the future, they could roll Characteristics for new characters.
Matt Ryan
Rolling dice to develop stats! 
Talk about Old School!
If people just want to game and have fun, that is not worry about the perfectly built character, rolling dice certainly speeds up the process so you can spend less time building and more time playing. Congrads on the quick improv!