Continuity mistake: At the beginning when the helicopter lands, the helicopter's shadow goes out toward the rear right of it (from the pilot's perspective), but when Sue grabs her bags and walks to Walter, the shadow is now cast to the front right. Since Sue got out and was standing next to it, the helicopter did not move in the meantime. (00:03:15)
Continuity mistake: When Dundee enters his NY hotel for the first time, he is smoking and puts the ashes on top of his hat because he cannot find anywhere else to put them. In the next shot, the ashes are no longer there.
Continuity mistake: When Sue, Mick and Richard are in the restaurant, first Sue is holding her menu, the next shot she is holding Richard's hand, the next shot she is holding the menu again.
Continuity mistake: As Mick gets up after Nevil leaves, he has a white tin cup in his hand. A second later, after the angle change, it's suddenly gone.
Answer: It is a party thrown by one of Sue's socialite friends, presumably the husky-voiced woman whose crotch Mick grabs. The party is not "full of hookers", Simone is there as the hired escort of the old man she is dancing with, who Mick naively believes to be her father. We can assume the old man paid for Simone to be his date to the party and likely for some time alone with her afterwards. There is no stated reason for why the party is being thrown.
BaconIsMyBFF