What was Gossip Girl Leighton Meester for Halloween? A rock star! The actress, who also moonlights as a singer, was invited to jump onstage Saturday night with Weezer at the Hammerstein Ballroom in NYC and duet with Rivers on the band's new single "(If You're Wondering If I Want You To) I Want You To."The lucky girl sang her heart out in an as-yet-unidentified costume — and oddly enough, Nat Wolff from the Naked Brothers band also joined in on guitar.
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