I've just been having the same issue actually and was coming to the forums to see if anyone else was dealing with this too. It's the same for myself, where I can see the fireteams on the list and even go into them and see OTHER people joining them, but the moment I try to click on which class I want to use in the fireteam and join, nothing happens. It's like the join button itself is just broken, so you sit there unable to join at all. Would like a fix as well lol