The bride-to-be, 29, shared a beautiful photo on Twitter in which she and Diana Ross' son Evan look deeply in love.
"What an amazing engagement party with all of our loved ones! We gettin' married :)," posted Ashlee, next to the ethereal snapshot.
CLICK ON PHOTOS FOR FULL GALLERY
Ashlee shows off stunning engagement ring - see the pic
In the photo, Ashlee and Evan are pictured gazing into each other's eyes and look as if they're about to kiss, as they share a tender embrace in front of their nearest and dearest.
The theme of the night appeared to be all things vintage, as the blonde beauty donned a flowing champagne coloured dress, which featured billowing sleeves and an embroidered floral pattern. Ashlee completed her show-stopping look with a sequined feathered headdress.
Complementing his fiancée's style, Evan wrapped up in a white fur coat and wore a simple white top underneath.
The couple's famous guests, who included Evan's mum Diana Ross - who was celebrating her 70th birthday - and Ashlee's sister Jessica Simpson, looked to be having a whale of a time as they posed for photos.
Beautiful siblings Ashlee and Jessica — who is also getting married later this year — cuddled up for a sweet photo. Older sister Jessica matched the host of the party in a sequined mini dress which showed off her svelte figure.
Jessica and her fiancé Eric Johnson are set to marry in a Los Angeles ceremony, after having ruled out Italy — a favourite destination with the couple — earlier this month.
Actor Daniel Booko also attended the engagement party, and was quick to share a snap of the lavish wedding cake which, like Ashlee's look, featured delicate feathers. The vintage-looking toppers "A" and "E" were carefully laid on top of the white cake.
"Celebrating the love of these 2 the other night! Congrats @realevanross missbananahammock !!," he captioned the photo.
Ashlee and Evan, 29, announced their engagement in mid-January, when they posted identical snaps of themselves on Twitter. "My baby love and I are ENGAGED!!! Hallelujah Hawaii !!!!!" wrote Ashlee referencing Diana's famous song, her huge engagement ring proudly on display.
This will be the second time that Ashlee has tied the knot. She was previously married to Fall Out Boy rocker Pete Wentz, with who she has a five-year-old son Bronx.