Check out the party snaps below.
Aniston and Pitt started dating in 1998 and were married two years later in Malibu.
"Jen and Brad have remained friends and have spoken with each other quite a few times since they both became single", a source told Entertainment Tonight. The relationship dissolved after he fell in love with then-co-star (and current estranged wife) Angelina Jolie, 47, on the set of 2005's Mr. and Mrs. Smith.
During the summer Jen admitted that she wasn't heartbroken following their split and has stayed good friends with Justin.
Having been together for nearly six years, the couple announced their separation in 2017.
"Brad, for long, was a very important part of Jen's life", the source added. "She was busy making the rounds and excited to see so many people near and dear to her". "Neither of them is pining for each other but instead, finding themselves again".
"He was invited, so why shouldn't he go?"
The baben' actor reportedly snuck out of the party early so as not to make too much of a fuss and Aniston gave him a buzz the next day to thank him and see if he enjoyed the event. Despite this, the "Friends" actress still had a huge smile on her face after celebrating with her pals over the weekend. The star of the "Friends" was 50 years old Monday and many celebrities shared their wishes online.