Yeah, the only co-op is Horde-mode style Invasion and Vs. Both modes support up to 10 players, but I haven't had any interest in messing with it. I will be playing through the campaign again to see if there's an alternate ending. There's some major plot points that seem to be based around who is in your squad at the time, so i'm interested in seeing if mixing it up leads to different scenarios. Would definitely add a lot of replay value to the game.