It hadn't been hard for Jon to decide to come. He's here for Robb more than for any other reason, but he came to support his brother, and that's what matters.

He mainly stays on the edges of the revelry; he's not familiar with the music that's playing, and none of the dances he knows go with the songs. He's fine with just watching, though, and at some point during the evening, he'll go in search of his brother or Sansa, and is otherwise easy to find.