JULIA is there because she lives there but also because she needed a night of relaxing and having fun, and Margo promised to get her fucked up in a good way so she's there for it. She's dressed on theme in a very Julia-esque way but I'm never the sort to go find specifics unless they're ridiculous.
She would have been friendly and maybe super friendly as the night went on thought not quite on Fen level.
Living there, she has the added bonus of just going upstairs and crashing when maybe she's had too much fun.