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, Hollywood Life reported.
Theroux popped the question on a romantic dinner date to celebrate his birthday at a restaurant in August.
Aniston, 43, previously said that the proposal was a complete surprise.
"It is hard to pull one over on me, where I'm not like, 'Hmm, you're acting really strange.' It was ... A very covert operation," she said.