The new system has led to knew kinds of griefing and exploiting. There's 3 boxers who will que up 3 different dpsers and get in your instance with all three - and then proceed to play only one of them with the other 2 on follow. There's people who will get inside your dungeon and just put someone on follow or afk the entire time, leeching xp and not helping at all. There's even people who will que up for roles they can't perform and explain it to their dismayed party - "I just wanted a shorter que time, can someone else tank/heal?". There's even jokers who will run around your dungeon nude, pulling everything they can just to try and get the group killed.
So how can you deal with these party crashers? Kick them out, of course. The issue is if Johnny McGriefstein is in your group, you can't kick them until you wait 15 minutes... Which is incidentally the same amount of time you have to wait if you leave a random dungeon to que up again. So it's sadly usually in your best interests to not even waste time with the bad apples and just leave yourself, hang out in Dalaran until your timer is up and try again.