They got married in July of 2000 in a jaw-dropping Malibu wedding, before finalizing their divorce in October 2005.
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.
"Happy Birthday to this fierce Woman".
Another source told the Entertainment Tonight website that when Pitt left the party, the hotel staff and his security team helped ensure that it was hard to take photographs of him.
Sneaky paps captured the actress fumbling down the stairs, presumably after one too many bevvies, as she made her way to the exit.
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Tensions rose when opposition leader Guaido declared himself acting president on January 23, a move which was supported by the US and many European and Latin American countries.
Here's hoping Aniston had a wonderful time on her big night.
Among the guests was ex-husband Brad Pitt, multiple United States media outlets confirmed.
Earlier in the evening, Jennifer, who turns 50 on Monday, stunned in a strappy black jumpsuit and heels as she entered the historic hotel, which is reportedly her favourite Hollywood hotspot. "That included Brad. The party was a celebration of Jen's life".
Despite various reports about whether Pitt and Aniston are still friends, his appearance at her party put the rumours to bed once and for all. Brad genuinely wanted to be with Jen on her special day. "We are two best friends who have made a decision to part ways as a couple, but look forward to continuing our cherished friendship".
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He created one chance, but it came from a corner kick, not open play, and had one blocked shot from some 25 yards. There's still a long way to go but we want to stay there. "I understand the fans", said the Italian.
She said: "I don't feel a void".
'My marriages, they've been very successful, in personal opinion.
And she continued: 'Sure, there were bumps, and not every moment felt fantastic, obviously, but at the end of it, this is our one life and I would not stay in a situation out of fear.
Not long after their split, Pitt started dating Angelina Jolie.
Other stars in attendance at Aniston's 50th birthday party include: Katy Perry, Ellen DeGeneres, Reese Witherspoon, Robert Downey Jr.
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FILE PHOTO: Jailed Bahraini footballer Hakeem Al Araibi leaves Thailand's Criminal Court, in Bangkok , Thailand February 4, 2019. Mr Araibi's supporters said he should be freed and is protected under his status as a refugee with Australian residency.