AUTHOR'S NOTE: The mod does not add co-op. It adds a multiplayer mode with no NPCs that encourages PVP play. The team behind it has no immediate plans to add NPCs to this mode. Thanks to everyone who let me know otherwise. Despite the lack of cooperative play, this is still a really amazing feat by regular people who really just enjoy playing the game. Looks like the co-op dream will need to wait until (hopefully!) Just Cause 3.