The couple became engaged last Friday (August 10), when Theroux popped the question on his birthday.
The Tropic Thunder scribe was spotted dining with Aniston's friends Laura Dern and Courteney Cox prior to the engagement, according to Us Magazine.
"Everyone seemed to be having fun," a source said.
Brad Pitt, Jennifer Aniston's former husband, is said to have congratulated her on the engagement and jokingly asked her if he should keep a date free for the wedding.
Pitt himself is engaged to Angelina Jolie, who he will reportedly wed in the near future.