Maybe I'm missing something, since I've only done Diadem once via the PF and I wasn't the leader, but there doesn't seem to be a way to replenish a PF group when people leave. This lead to a situation where we had a few people leave, and those who remained weren't doing enough to get credit for chests. I suppose you could get people to promise to stick it out for 90 minutes, but between unexpected disconnects and people just deciding to bail, it seems like the current design makes it likely that one person is going to, in effect, get locked out because someone else decides to leave.