It's best to avoid playing with friends for the time being.
Call of Duty players are experiencing multiple crashes while attempting to play Modern Warfare 2 on PC.
PC players appear to have an increased number of crashes on MW2 compared to those playing on Xbox or PlayStation during the first few hours of the games launch. Reports of consistent crashes began popping up on Reddit and Twitter shortly after the game went live on all platforms. The crash reports vary, with some saying they cant get past the main menu and others experiencing crashes after getting into a game.
Infinity Ward, the producer of MW2, has attempted to address the issues on Twitter and has released patches to help combat the issues. But players are still experiencing multiple bugs which are making the game unplayable for many in its current state. Modern Warzone on Twitter reported that players who were trying to queue up with friends were experiencing more crashes than those who were attempting to play by themselves.
With the game freshly released, it is understandable that there would be server issues. Thousands of players in North America are currently flooding the servers all at once, slowing down the process. For those who are trying to play with friends, the issue seems to be rooted in getting all the players, regardless of the number, from the same party in a lobby together. This could be due to a number of issues with different servers and connectivity, but for the time being it seems best to play solo and wait for further updates from Infinity Ward.