As far as I know, there's no one click button to make that happen, but doing it is easy enough.
In the event, just create a random node and add 20 random step. Now, create 20 join active group node(or whatever group node you want), put a new combatant in each one, and link each one node to each random step. Now, just call the event and it'll automatically chooses a combatant at random to add into your team.