These are some headcanons of the Gryffindors at the party.

Weasley Twins: The DJs

Ron: Eating all the Food at the Buffet

Harry: Just Chilling on the Couch

Hermione: Undercover Party Animal

Neville: Doesn't Like the Party so Much

Ginny: Dancing. Like. Crazy.

Oliver: Trying to sneak out to go to a Quidditch Practice

Seamus: Sneaking Some Irish Whiskey

Percy: "SHUSH LISTEN TO ME- GUYS GUYS I'M HEAD BOY-"