Nearly a week after turning 21
, Selena Gomez
celebrated the milestone Saturday night with plenty of friends – and a surprising familiar face.
Gomez – who shares her July 22 birthday with the newborn Prince George of Cambridge
– hosted a gypsy-themed bash at Malibu's Revolve Beach House. On the guest list: Julianne Hough
and brother Derek, Nina Dobrev, Ashley Benson, Jaden Smith and celeb stylist Riawna Capri.
"When Selena's song 'Birthday' came on, all of the guests gathered on the dance floor and took shots of chilled VeeV," a source tells PEOPLE. The evening, which was filled with dancing, good friends, and plenty of drinks, got an unexpected visitor around midnight when the Spring Breakers
actress's on-again, off-again beau Justin Bieber
"Justin arrived at the party in his leopard-print Audi R8," a different source says. "He was casually dressed and carried a single rose."
When Gomez saw him she "started to cry," according to the first source. "Justin and Selena went into the house to talk and were inside for a long time before sneaking out together."
But aside from Bieber's unexpected appearance, the ladies were dressed to party and showed off their midriffs – after arriving at the over-the-top celebration via party bus.
One of the night's highlights: a snake-charming dancer.
As for the birthday girl, the second source says she left the party with Bieber, 19, around 2 a.m.
With reporting by MELODY CHIU