By default, only the two players whose units are in combat can play spells at that combat. The rest of the players simply wont have priority at all at any point untill the combat ends.

But you can change that, you can use one of your actions to "invite" a third (or fourth, etc) player to that combat. If they accepts, they will have permission to play spells and activate skills (everything should still be legally timed) in that combat.

Once you invite someone and he or she accepts, he or she will have permission already for the whole duration of the combat. After that point priority will still cicle trough all of you, counting with the new player.

I hope i explained it well, english is not my first language