Acorn Minor.
You need to have the excate same files as the person who is hosting the game. If you have any different mods installed on your install to the person who is hosting you will CTD.
Make sure that you and all the other guys who want to play together have the same install and you will no problems
Us guys that play MP on here use the MP match up thread to let people know when a game is up and what mods they have installed.
I hope this helps