Dear Survivors ..
Yes, there are still problems in the friends list for invitation or play together, but there is another way for you to play together. I have two computers and bought two copies of the game. Follow the procedure below:
1 - On both computers run the game and select the online cooperative campaign mode option.
2 - At the same time, tap Ready to search for a session.
In this way it seems that the game looks for who is next to you geographically, at least I think. My friend and I always manage to play together this way, but sometimes it does not work out and you have to leave the session and re-dial again until you are together in the same session.
But the correct one is to FOCUS solve soon this question of the list of friends.
Sorry if my English is not very good, I'm Brazilian.
Hugs to the Survivors ...