Poptopia came to a rousing finish with a Sunday-night performance that seamlessly linked the past and present. After the Groovie Ghoulies' frantic show, the bulk of the crowd--suffering under California's antismoking law--rushed outside for a synchronous smoke. For a minute, it looked like everyone was going home. Twenty minutes later, the smokers returned as one, and Matthew Sweet and Tommy Keene took the stage, fronting a quintet simply billed as Groop. For one night, these two noted pop songwriters could pretend to be part of the cover band at the prom, as they tore into classics like "The Kids Are Alright," Big Star's "In the Street," "Paperback Writer," Badfinger's "No Matter What" and The Kinks' "Where Have All the Good Times Gone." For a few moments, the stars were the fans, the fans were the stars, and no one could tell the difference. That's when Poptopia lived up to its name.