We've been dealing with this one for the better part of two years, or maybe even longer. It seems to happen more frequently when people join the raid in a pre-existing group. Occasionally, when you go to move someone from the "ungrouped" portion of the raid into a group, the raid leader and about a dozen other people will all instantly lock up, and you have to control-alt-delete to terminate the process - it never crashes on its own. When it does happen, we almost always have to have everyone disband that raid and start a new one or we will have this problem on a repeated basis as long as we keep that raid. I have not been able to definitely prove reproduction steps and would welcome any pointers on things to try/eliminate for testing purposes.