The 'Pieces of Me' singer, 29, and the budding actor, who got engaged in Hawaii in January after only six months of dating, were joined by dozens of friends and family members, including the blonde beauty's older sister Jessica Simpson, to toast their upcoming nuptials in Los Angeles.
The party was extra special for the couple as Evan's famous mother, Diana Ross, was also celebrating her 72nd birthday.
Weight Watchers spokesperson Jessica, 33, posed for a photo with Ashlee, who was wearing a bohemian, free-flowing tunic top and a flapper-style feathered headband, and posted it on Instagram with the caption: ''Celebrating my best friend and sister @missbananahammock @ashleesimpsonofficial. I love her so much!''
Ashlee later shared an image of her and Evan, 25, kissing at the outdoor soiree on Twitter, writing: ''What an amazing engagement party with all of our loved ones! We gettin' married (sic)''
Despite hosting a huge party for their loved ones, Evan recently revealed he would love to elope with Ashlee, who has a five-year-old son, Bronx, with her ex-husband Pete Wentz.
The upcoming 'Hunger Games: Mockingjay' star said: ''Honestly, we could go and get married right now, wherever. We just want to be married. We're in love.
''This is the most incredible time I could ever think of in my life and I'm young, but the same time, I've lived a different life than most and I can't think of a happier moment.''
But he later hinted they were planning a much bigger wedding, as he confirmed his mother was playing a crucial role.
He said: ''She's involved in every piece of our wedding [and] engagement party.
''She's involved in all levels of my life. She's one of the most incredible mothers ever.''
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