So I guess I am missing something obvious, but right now I am unable to create a party in game with my friends from steam. I invite them but then it kicks them to the entry screen and says "unable to join session" after they accept the invite. What exactly is the process for creating a party to join games so that I can play with my friends? Is this an in game bug? Looking for some help here, would be much appreciated. I noticed there are clans that are joining as a unit in game so it must be able to be done?
Either you or the friends you are trying to invite have a strict NAT type (type 3). This complicates matchmaking and can cause players to get kicked from parties. Check at the bottom left of the screen before finding a match and you will see what your NAT type is.
Go to www.portforward.com and find Black Ops 2, find your router, and follow the steps. Depending on the brand/model of your router, this could solve your problem. If you are unable to open your NAT this way, you might need to invest in a better router.