As seen in Ginny's post, Quidditch party Saturday. I would say the following are automatic invites.
Ginny Weasley (Guest Harry Potter)
Connor Summerby
Oliver Wood
Katie Bell
Marcus Flint
Eddie Carmichael
A few others may be able to work the event (Cupcakes? Music? Security?) but for the most part anyone else in that party would have to be a date of the above mentioned. Tell me if I missed someone