You need one or more friends with the same SCP build. Then you can play the main FS2 campaign in cooperative mode. Note, however, that your progress is never saved, but it doesn't matter, because you can select any mission you like. Just write down in which mission you were when you stopped playing last time. Also remember to check when the "loop" mission are supposed to be played.
Setting up multiplayer game is easy, one of you hosts, and the rest of you join his server. You can add servers from the options screen, if I recall correctly. You just need the hosts IP.
