Demon Stone or X-Men Legends? Not sure if X-Men has the AI coop though. Pretty sure it does.
Yup it does. If I remember correctly, a new player can join/disconnect the game at any time and take over one of the coop AI spots. The only downside is that you have to play through a level or two in order to get to the point in the game where you have more than one person in your party top play with. Not a dealbreaker, but just be prepared to play single player for 15-20 minutes before you're able to break out the coop.