Nyx is there of course! He'll do the knife/axe throwing thing and might be showing off a little on the first. Just a little lol. He'll be sure to chat with whoever he knows, people he doesn't too. Just enjoying the festivities and his birthday.
Natasha would have gone. She'll do some knife throwing and having a few drinks. Will stick mostly to her group but will talk with whoever else is around.
Gladio probably came with Noctis because of Nyx's bday. Never got the hang of fancy knife throwing but can throw a mean axe. He's friendly enough and would chat up whoever if anyone wants things there!