Tobias attended as the ward of Jon and Daenerys. He would have either kept close to them or was off by himself as he isn't too good with people. He would have stayed for two hours then hang around the rest of the evening as a hawk.
Lyanna was of course there. She would have been so happy to see them get married.
Robb was there, likely with his wife.
Theon would have made an appearence. He would have stayed off outside of the crowd but would have a good time.