I go into multiplayer, set it to 'invite only' and invite my friend via steam. He says that when he accepts it says that I'm not currently playing on any game server. We tried it the other way round but got the same result.

Does anyone know what I'm doing wrong???

many thanks
recoil

I have the exact same problem. Does anybody have any idea on how to fix it.

No idea but I need help on this too.

I thought was maybe some ports that needed opening. I've tried all those that people suggest in other forum posts, and nothing works...

I have no friends, lol so I haven't been able to test it.
Most of the time there is no one for online play when I look or try to play the online parts.