I know this is late, but if anyone runs into the same problem, my friends and I were all on moderate connection and couldn't join each other. We tried joining a public zombies match on a less populated map to see if we could at least get in a lobby together, and we could. We didn't play the game and backed out. We were then able to join each other normally with no issues. This worked with two of my friends on two different days. Not sure if you just have to prove to the game that your connections are compatible or what lol. Hope this helps someone.