Make your two squads. Give everybody a 100% probablility of presence except one of the squad leaders. Make his probability 50%
ooooohhhhh I can't quite remember the next bit ...... in the init field of the 50% leader write
present1=true
and in the Condition of presence fields of all the other soldiers (both groups) write
present1
That way, if the leader comes into existance all the others will too, and if he doesn't, they won't.
I haven't got my OFP at the moment so I can't check if this is exactly it, but try it and see.