This has been happening to me for awhile on Xbox hosted sessions. It works for awhile then suddenly you can't join without an invite from someone already in the server you want to join. I heard rumors that there's a patch supposed to be coming out to fix it but not sure how much I believe that. It makes me not want to play anymore since I won't do official because the stats are so low. The only ones that I can always join are on the list of servers in the start screen but even a lot of those dont get enough population so they end up disappearing.