While the appearance of the no-parking signs and Aniston's hairstylist got the rumor-mill churning the pair was preparing to tie the knot in a surprise wedding, they instead opted for a casual afternoon by the pool.
Among the famous faces in attendance: Dax Shepard and Kristen Bell, Isla Fisher and Sacha Baron Cohen, Jimmy Kimmel, Tobey Maguire, Jason Batemen, Ben Stiller, Ellen DeGeneres and Portia De Rossi, John Krasinski, Don Cheadle, Lake Bell, and Will Arnett.
"Justin looked very happy and like he was really enjoying his birthday and showing off their new house," a source said.
"Justin was hanging out with Scott Campbell and Dax Shepherd at the beginning of the party," the source continued. "They were having a great time laughing and joking around. Jen was nearby talking to Mandy Ingber and Isla Fisher. She was hugging her friends and talking closely with them most of the night."
Guests in the living room gathered around a blazing fire and hurricane candles and white lights lit up the property as the sun went down until the party wrapped up around 11.
Celebrity Couples of 2014 [PHOTOS]