Yes i am on pc. Thanks very much OldTask. that info was right-on! did exactly that and now can play COOP with best mate.
just now need to work out how to do that using my own Battlefield3 Server. once i have my server all sorted with all maps and gam types, will be avail for CheatHappens Members to freely come and play.
You can't play co-op on a rented/bought server. Player who invites another player is the host, meaning it's Peer2Peer type of connection (Games for Windows LIVE style).