It's not really come up in my group much, but I've got a house rule that any more than 6 minions in a group would only add Boost dice, not skill ranks. And it's literally only came up once; an enemy had formed a squad of 7 minions and got a triumph, so that's the ruling I went with for their attack. But you know, that was kind of a spur of the moment thing, and haven't really thought it through too much.
RAW, you don’t even add the boost dice for the additional members. Having more than six members just means that you can kill more of them and still have their rolls maxxed out.
If it’s a seven member minion group, they get exactly the same roll as a six member minion group. However, if one member of each gets killed, then the former seven member group still makes the same roll as they did before, while the former six member group is now a five member group, and they aren’t as effective as they used to be.
This makes large numbers of large minion groups pretty deadly, because they can take a lot of damage for a long time, and still be every bit as effective as they were at the beginning.
Of course, it is your game, so it is also your rules.
I do kind of like the boost dice house rule, although I think I might cap that at five boost dice.