I was invited to this mission when my friend was at D5. But I was AFK and didn't join until D7... in the boss's lair. Not sure you should be able to join the boss level, aren't they usually restricted?
Mission: Shadow of the Beast! - Joined at D7
ven, 02/10/2012 - 12:02
Culture
If you get invited earlier you can go in. That's how it works in the regular server. I've actually once jumped into a party right as they kill the twins and I got 1 token.