Is it still possible for friends to join enemy team in QP?

Just had 2 QP matches in a row with the same person joining as spectator, then loads into my team, picks mei and icewall griefs us incessantly…

Does the “join game” option still allow someone to back fill into either team when someone leaves or was this just some really weird stuff going on?

As long as you don’t group with the people in the game yes. If you’re grouped it will only slot you in on the same team. Ungrouped I think you can join the enemy. It’s happened to me, unless they’ve patched it.