led guests in a rousing rendition of "Happy Birthday" while celebrating Nicole Richie
's 30th birthday
at the Four Seasons Beverly Wilshire in Beverly Hills along with celeb pals such as Mary-Kate
and Ashley Olsen
and Samantha Ronson
. The birthday gal got a Samsung Galaxy S II, which she used to snap pictures all night. After enjoying some cake, the bash turned into an all-out dance party, during which Ronson took the mic and challenged people to a dance-off.