This error has been happening the past few days. Whenever I am playing a mission, one friend can join and I will be fine but when a second joins it causes not only the game to crash but steam as well. It has happened with more than one person on my friend's list and I'm believing it to be a problem with either Steam or Java. I have disabled the Steam overlay while in game and that hasn't done much to solve the problem.
Mind stating your computer specs, java version and uploading the contents of the launcher.log in your game folder?