Jennifer Aniston & Justin Theroux Giggle Together at Cheateau Marmont

Jennifer Aniston was in a bright, sunny mood while hanging out with boyfriend Justin Theroux at West Hollywood's Chateau Marmont, an onlooker tells us. The couple seemed "content and present with each other" even as they were hanging out with a big group of people in the back area of the hotel's patio.

Aniston was bundled up for the weather, sporting a black wool peacoat and black pants, while her actor beau was in a leather jacket and gray T-shirt.

Theroux also greeted fellow revelers on the patio, sharing a hug with fellow guests like Zac Efron, who was there to hang with some guy friends. But when it was time to go, he only had eyes for Aniston, and the two giggled together as they left. Says the observer: "She and Justin were beaming and seemed enamored of each other."

Also at the Chateau that night: Eva Longoria, who was celebrating her 37th birthday with friends; and an energetic Lindsay Lohan who arrived at the hotel after midnight with one guy and a blonde female friend.