Girls get together and have sleepovers all the time. If she invites guys, they can stay over that way.

[ He'll point towards the area where, if Hak's been wandering around enough, he'll know there's the furthest location that you could house someone for the night while still being on the temple grounds.

Meaning they'd have to go past Shizuo's room (along with some others) before getting to Yona's. It's probably why he's so calm. ]

She hasn't done it yet though.