In the beginning after boot camp you will have to get Carlos out of an enimies camp. When you get him out he will join your team. Also after that level you will have to do a rescue mission to rescue Erika (I think thats her name) from another campand kill all of the enimiesto get her. Some of the people will just join you. Trust me on this I play this game almost every night. But I'm not done with it yet and the game is kind of hard to some people. Well hopes this helps you out.