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

Related: Justin Theroux wants to start a family with Jennifer Aniston

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."

Related: Jennifer Aniston engaged to actor Justin Theroux

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."

Related: Alyssa Milano denies dating Justin Theroux

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".

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