Model Gigi Hadid, 19, helped her Australian pop singer and actor boyfriend Cody Simpson celebrate his 18th birthday in Malibu, California on Jan. 11.

Us Weekly reports famous pals including Simpson's sister Alli Simpson, Justin Bieber, Jaden Smith, Miley Cyrus and her boyfriend Patrick Schwarzenegger joined the couple at the casual house party. As guests gathered inside the house, Cyrus reportedly presented the breakout singer with a pizza-themed cake.

Hadid looked sexy in a black leather crop top and matching skin-tight pants, while the birthday boy wore a backwards hat, black T-shirt and jeans.

"Birthday boy got thrown in the pool," the Sports Illustrated model said in a caption of an Instagram picture with Simpson, who was soaking wet.

Hadid and her beau actually started celebrating Simpson's birthday on Jan. 1  at the Nice Guy in Los Angeles with several friends, including Selena Gomez and Bieber. Gomez and Bieber somehow managed to avoid one another. (Gomez was absent at Sunday's birthday bash).

HollywoodLife.com reports that Maybelline New York announced on Jan. 14 Hadid will be their newest brand ambassador. She joins the ranks of models like Christy Turlington and Adriana Lima.

"Being a Maybelline New York spokesperson means more to me than anyone can imagine; it doesn't seem real! It's the most incredible feeling and is a dream come true. I feel so honored and humbled to be a part of the iconic brand I grew up with, and I am very excited by this opportunity," Hadid said in a statement.

Hadid's modeling career has been rocking the high fashion creations of Marc Jacobs, Jean Paul Gaultier, Chanel and others on the runway. The cosmetics industry is a new venture for the Palestinian-Dutch beauty.

Print and television appearances are old-hat to the teen model. She has appeared in Sports Illustrated, VMAN, Vogue, Elle and Paper magazine, to name a few. She has made many appearances on "The Real Housewives of Beverly Hills."