Eddie Rickenbackers is a fun hang. Old motorcycles dangle from the walls and ceilings. Not too far from the convention center.
Review here.
Another idea is
Gordon Biersch
La Rondia, in the Mission district. Cheap Mexican food and pitchers of Margaritas. Loud.
Jules, would you guesstimate 20 to 30 people? More? Less?
More suggestions on the way.