Ok, as a standard action, they can affect 1 character within 30 ft of one of the group?
You have a point about the immediate action. Ok, let's do that.
How about if the spell targets or is cast within X ft of any of the misi?
That works. Stick with the usual 30 ft.?
I'll go for 60 ft tops. Space is big, but they should still have to get fairly close.
Oh, let's just go for 60 ft. That's a more common range than 50ft.
I'm fine with that.