This is just to be able to play the game with more than 1 other person in the co-op campaign mode.
2-4 people as any more would cause games to be far to unbalanced.
Perhaps look at a system where these people dont start as allies, just a thought.
Please look at a bigger multiplayer system. No more than 4 people.
Joke: Nerds have more than 1 friend sometimes XD