Taylor Swift got in on the act at The Darby the other night when she and her band dined at the West 14th Street supper club. Darby house vocalist Ron Grant spotted the singer in the audience and in vited her onstage to join him for a Marvin Gaye duet. The songbird eagerly accepted and then, as an encore, dragged her band up to run through an impromptu version of the hit “Love Story.” Spies said Swift re ceived a five-minute standing ovation from the crowd before returning to her meal by chef Alex Guarnaschelli.