Vanilla Arma 1.90 and before
When a JIP choose the marksman slot in a vanilla weapon squad group, the Team leader (first player) can't order useful moves. The regroup order seems to work but any other move (arma menu) will make the AI units resuming some weird formation around the marksman who doesn't make any move (because he's played)!
Place a weapon squad (BLUFOR tested) on map
Make all slots playable
Run preview or mission in MP
Choose the TL , order what you want to your group... fine!
Ask a friend for JIP and ask him to choose the marksman slot (not tested all configuration but here it's 100% guaranteed)
Order your group to move. The AIs are grouping around your friend (or you if you regroup them) but they don't go to any move point anymore.
Easy to reproduce
It's just an unacceptable mutiny!