Here is what my friend tried to do.
If you have any questions for him. His name is Vnoqe-_-. He will be glad to answer your questions.

How do I do that? Also, no I didn't try that (if you can help me out with that I would appreciate it.)

Hi there. Does anyone know why my friend can't join my private match to play some zombies? (Is there something up with plutonium I don't know. Also, I got everything set up and doesn't allow him to join). On top of that, when I try to join him it only says he is online and same goes for him to my lobby.