Pre-party, a source tells Us Weekly that the ladies got ready inside (Aniston's hair stylist Chris McMillan was on hand to style the "We're The Millers" star), while Theroux hung out with his guys outside.
"Several of Justin's friends spent the afternoon at the house hanging out by the pool with him and celebrating his birthday," says an eyewitness. "It was a relaxed afternoon and they were all in the cabana area having drinks and listening to music like Kanye West and The Kinks."
"Most people were dressed casually in jeans and T-shirts," a source tells Us. "Justin was dressed casually in a black T-shirt and black jeans, and Jen had on a short green skirt and long sleeved top."
While the actress, 44, chatted with her friends, mostly sticking to gal pals Fisher and Meyer, the birthday boy -- who received gifts from Cartier and Barney's -- spent a majority of the night with Shepard and Kimmel.
At the celebration, which fell on the one-year anniversary of the pair's engagement, guests enjoyed a relaxed and fun atmosphere.
"All of the windows and doors were raised to give the house an indoor outdoor feel," says the source. "People were admiring the city views as the sun went down. There were candles everywhere and a fire pit was lit."
Click on for more photos from Jen and Justin's romance...