There's something I learned last month that might be interesting to know. Say you queue for expert roulette, and you get Sastasha HM. Let's say the healer drops because they hate the dungeon. If you have a healer in your FC queue for Sastasha HM in progress, they won't get your group. If, however, they queue for Expert Roulette join in progress, they might get you. Queues for specific dungeons are apparently different from roulette queues.