I've also experienced this issue every single time I've been placed into Orbonne Monastery. It seems almost like the game is putting a cap on how many players can be in an alliance at the same time after people start leaving. The last time I got Orbonne, for example, alliance B lost 3 people after the first boss. Several people asked their friends to join as a healer and none of them could join. All of them confirmed that they had "join party in progress" checked. After a while one of the dps in alliance B left and immediately got filled with one of the healers who had been trying to queue. After that they were unable to gain any people again, as if they were capped at 5. The whole time our alliance was shedding and gaining new members.