Hey!
You should be able to just add your friends on the forum site, and then once they add you bacxk you go into custom game for Zombies and invite them. To accept an invite, go to Zombies then online then click the friends tab in the bottom right, then click the party invites tab and accept it or vice versa.
If online play does not work for you, the host should Port Foward to the port "3017". This worked for me.
Have a good day!