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Adam Gregory, Jessica Lowndes, Dustin Milligan, Tristan Wilds

Albert Michael/Startraks

90210 bad girl Jessica Lowndes just turned 20, but she knows how to throw a good party.

The actress had a posh masquerade bash on the rooftop of the Thompson Hotel in Beverly Hills, and costars Adam Gregory, Shenae Grimes, Jessica Stroup, Dustin Milligan and Tristan Wilds all came out to celebrate.

Jessica and Adam, who are reportedly dating, even donned matching silver masks for the fete. While there was no blatant PDA, the two were by each other's sides for most of the night, even leaving together around 2 a.m.

Although the party was sponsored by Tava, a sparkling nonalcoholic drink, Jess was seen sneaking libations.

After ordering a glass of white wine, she quickly dumped it into a rocks glass so it wasn't so obvious that the underaged starlet was drinking. Nice try, J.