Justin Theroux and Jennifer Aniston threw an engagement party at their Beverly Hills home on Sunday (02.12.12).

The ‘Wanderlust’ co-stars celebrated with close friends including Demi Moore, Ben Stiller, Tobey Maguire, Ellen DeGeneres and Portia di Rossi at the low-key bash, almost four months after first announcing their engagement.

Justin popped the question on a romantic dinner date to celebrate his birthday at New York City restaurant Blue Hill in August.

The couple then enjoyed a weekend of private celebrations before Justin’s rep announced the news in a gushing statement.

His publicist said “Justin Theroux had an amazing birthday on Friday, receiving an extraordinary gift when his girlfriend, Jennifer Aniston, accepted his proposal of marriage.”

Jennifer previously admitted the proposal was a complete surprise.

She said: “It’s hard to pull one over on me, where I’m not like, ‘Hmm, you’re acting really strange.’ It was … a very covert operation.”

The 43-year-old actress also hit back at critics for suggesting her engagement ring is too big and insisted she loves it despite not selecting it herself.

She said: “I was impressed. I had no idea, and I had no idea what I wanted. He just knows what I like.”

She said her fiance “worked with the girls that I have worked with for years and years and years, so he kind of was picking the specialists’ brains. He kept it within a very small circle of people. It was one of the sneakiest jobs I’ve ever seen, actually”.

Article and image via BangShowbiz