I think its because the version of cfs3 that you have: if you have the 3.0(like me) you only can play with the people with this version. But there are problems, because i cant make a match: when i create one the people can't get in....
Problems of game

I'm using the fully patched version of the game. I have discovered that the problem lies in my USRobotics modem/router. The only way around it that i've managed is (after i've hosted a game) to reinitialise the modem. Once it's reset and reconnected to the internet i have no problem connecting to other peoples games.
I hope this post will help anyone else who's being driven potty by this problem.