Thanks, Garcia. I understand what you mean, and have done it that way. Rescue group has all units marked as playable, and prisoners are marked as playable. Once you rescue a prisoner, he joins the Rescue group, and then is respawnable for players in Rescue group.
But I don't like the Prisoners being selectable as Playable units at beginning of coop mission, as there is nothing for a player to do as a prisoner.
What I'd like is to have prisoners be Playable only after being rescued.
I saw a mission that did this once, where reinforcements arrived, then joined the players group, and were playable. But these reinforcements were not shown as selectable playable units at mission start.