I'd recommend using the chat and state that you're having connectivity problems. If you can never play with other players, I'm sure they want to hear from you.
I have tried to see what was wrong and tried to fix it many times with my friends, when joining games, I have only tried joining a few games on MP though, just to see if it maybe was me and my friends problem (my friends can play without problems, unless I'm in the game) moment I saw that wasn't the case I haven't joined any MP games anymore.
Barring that, any details you can provide regarding what kind of connection you have and what hardware you're using would go a long way.
I'm from the Netherlands, 10mbit connection afaik, behind a router though which is downstairs.