The singer dressed like a flapper for the gathering, sporting a feathered headband while her husband-to-be wore a furry jacket.
"What an amazing engagement party with all of our loved ones! We gettin' married," the 29-year-old singer tweeted after the event, along with a photo of herself and her fiance about to kiss.
Simpson's sister, Jessica Simpson, was among the party attendees. The Weight Watchers spokesperson also shared a photo from the soiree on her Instagram account.
"Celebrating my best friend and sister @missbananahammock @ashleesimpsonofficial. I love her so much!" she captioned the pic.
Wednesday's celebration comes two months after Simpson announced she was engaged to Ross on Twitter. This will be the second marriage for Simpson, who previously tied the knot to Fall Out Boy rocker Pete Wentz. The couple divorced in 2011 after three years of marriage. The two have one son, Bronx Mowgli Wentz, born in November 2008.