The insider added the actress shows her appreciation for the cast and crew, which includes Witherspoon, Steve Carell, Billy Crudup, Mark Duplass and Gugu Mbatha-Raw.
A surprise show was ex-husband Brad Pitt, confirming more than a decade of rumours that the pair remained close friends following the break down of their marriage in 2005.
Theroux, 47, posted a birthday tribute to Instagram on Monday in honor of Aniston's 50th birthday. She debated back and forth with friends if she should invite him.
A few of Aniston's friends gave a glimpse inside the party, thanks to a fun photo booth set up inside.
"Jen was the first guest to arrive around 8 p.m. wearing a black jumpsuit with her hair down", a source tells E!
"Since they split, her friends believe their plan to move forward in life without one another was the best decision for both of them", the source said at the time.
Aniston and Theroux tied the knot in 2015, before they announced their split in 2018 after two and a half years in marriage.
"I don't feel a void". And when they came to an end, it was a choice that was made because we chose to be happy, and sometimes happiness doesn't exist within that arrangement anymore'. That same year, Pitt began a relationship with his Mr. and Mrs. Smith co-star Angelina Jolie.
Vaccinations jump 500% in antivax hotspot amid measles outbreak
The announcements come as a measles outbreak hits the Pacific Northwest, where more than 50 cases have been confirmed. The vaccine can reduce the risk of infection if gotten within 72 hours after exposure to the virus.