June 17, 2005 02:25 PM

Mary-Kate and Ashley Olsen may have had a relatively tame 19th birthday – they celebrated with cake, balloons and sushi at an L.A. restaurant Monday – but it appears Mary-Kate saved the hard partying for Wednesday and Thursday night at New York nightclub Bungalow 8.

Mary-Kate arrived around 1 a.m. early Friday morning with an entourage of 10 to 15 friends (but not her sister) for a night of wild dancing, champagne, shots, cigarettes – and more cake.

After heading to the bathroom with her tiny white minidress falling off her shoulders, Olsen stepped out clutching the hand of a short dark-haired man who became her dance partner (but not romantic partner) for much of the night. When the two of them reached the middle of the dance floor, they ran into Russian supermodel Natalia Vodianova. Mary-Kate vaulted herself into Natalia’s arms, and the model scooped Mary-Kate off the ground and cupped her rear-end as the pair exchange kisses.

Natalia’s husband, Justin Portman, appeared with shots for the group and gave a short toast. Mary-Kate had a glass of champagne, and she later donned a cowboy hat and grinded with girlfriends to songs including “The Seed” and “Get Right.”

When 50 Cent’s “In Da Club” came on (complete with the “It’s your birthday” refrain), friends gathered around Mary-Kate and two cakes frosted with roses were brought out. After she blew out the candles, the dancing continued, while a bodyguard stayed close to Mary-Kate’s side. The group made their exit around 3 a.m.

“School’s out. Summer’s here and they were having fun,” Olsen’s rep, Michael Pagnotta, tells PEOPLE.

As for Ashley’s whereabouts, Pagnotta says she’s currently in L.A. while Mary-Kate is in New York for a few days.

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