As one insider told the publication, "She debated back and forth with friends if she should invite him".
Reese Witherspoon, Katy Perry, Orlando Bloom, Ellen DeGeneres, Portia DeRossi, Gwyneth Paltrow, George and Amal Clooney, Sandra Bullock, Cindy Crawford, Robert Downey Jr., Diane Keaton, Barbara Streisand, Jason Bateman and Kate Hudson were also spotted at the hotel, but Theroux was not. She shared her love for Aniston on Instagram today, January 11, with pictures of her, Kate Hudson and Aniston from the party.
No Hollywood party is complete without a photo booth to capture memories of the big night, and judging by social media Jen's pals made great use of the facility at the Sunset hotel.
It was not reported if Aniston's second ex-husband Justin Theroux attended or was invited.
An eyewitness tells ET that Pitt arrived at his ex-wife's birthday party with his security team.
"Happy Birthday to this fierce Woman. We love you so much and you make 50 LOOK [fire emoji]", she wrote in the caption.
The "Cake" actress - who split from Justin Theroux last February after two years of marriage - recently insisted she still regards her relationships with both Brad and the "Leftovers" actor as "successful".
"The good news is that was probably the most - I'm choosing my words really carefully - it was kind of the most gentle separation, in that there was no animosity", he said.
She said: "I don't feel a void".
People reports Aniston and Pitt have been friendly, but not especially close, exes for some time, but are "over each other" romantically. 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'. They divorced in 2005 after Pitt was linked to his Mr & Mrs Smith (2005) co-star Angelina Jolie.