Thats BTW why i suggested (and really hope it gets to Larian) that our good companions who refuses to participate in Groove massacre, should ... well, refuse to participate in Groove massacre.

And in my honest opinion it should be quite easy to implement:
Once you click at gate contol ... your present good characters will confront you ... you either persuate them (and then they dont leave you after the fight) ... or you dont, and they stand against you in upcoming battle.[/spoiler]