CAITLIN'S Fandral is there because, well, obviously Hugh told him about it and even if Fandral probably should still be resting after that Wampa attack, he's feeling much, much better. He'll be happy to dance and flirt and drink and just generally do Fandral things with whoever, though he'll probably also be checking in with Hugh regularly.
Kady is there, though she's not dancing. She got pretty fucked up during the Wampa attack too, so she's hanging out with Julia and trying not to be too broody about Ethan disappearing while she was in the clinic.
Margo's there because fuck yeah, even if it's some weird little dance party, it's something to do in this place. A party is a party and she's going to take full advantage of that fact.
Sydney goes, mingles, and maybe dances a little? She's still not the most social of creatures.