I have been reading that many people are having issues with joining friends games on the ps4, pc and xbox. I have also been having the same issue. Here is what I know since I have been experiencing this problem. I can only connect to my room mate in my home. I have his xbox on the dmz and mine has all the ports for xbox one static to my xbox and both show a open nat. Room mate can join others games, I can not whether he is in that game or not. Everyone has there multiplayer settings in game to friends invite as well. When I try to join someones game from the main screen of dying light nothing happens from party chat or invite through xbox. But when I have my Party chat snapped I can see everyone's status go from "dying light" to "in your game", but there I am seating there looking at the main screen with nothing happening. Eventually after opening and closing the game with many invites later, I finally some how manage to get the game to come up from the main screen for me to join there game. Once it starts to load the game, I then press A and bam "this game session is no longer available" keep in mine that during this hole time it is saying that I am connected with the "in your game" status. I have had friends port forward with open nat, hard booted my xbox, hard booted modem/router, everything under the sun to get this coop to work. It would be great if the devs would acknowledge that there is a problem and at least say they know of the issue and are working on the problem.