I'm going to go with two men and two women to keep it balanced.

Balthier - He would be that suave guy who is friends with everyone and does little magic tricks. All the women want to sleep with him, so by inviting him to the party, I get bonus female NPC guests.

Barret - He's my security. Could be swapped for Ward, but Barret would probably have a go at karaoke whereas Ward would not. Nobody smurfs around with him there.

Selphie - She's the life of the party. She'd organise an impromptu band session and we'd get live music and she's just so excited and will probably be a lot of fun.

Terra - Can you imagine how exciting your party would get with a girl who can become half naked and pink? Goddamn. Plus she can do magic tricks. Real magic tricks.