Survivors can join your group in two ways. You rescue them on a mission, or you call out for them by using up a Radio. 

Each survivor in your group will use up a survivor slot. You can gain more slots by buying it with gold or buying bundles that contains a survivor slot. When you reject a survivor you gain a small amount of experience points and a percentage of the tokens that were spent on them.

You will only be able to recruit survivors of a class once you have unlocked them by playing the Chapters in the game.