About 40 people showed up Sunday evening at L.A.'s Eveleigh restaurant for dinner and what an ABC News source called "touching toasts" from folks on both sides of the marital aisle.
"Then everyone went dancing," the source said.
The couple was spotted leaving the Warwick nightclub late Sunday.
Simpson and Johnson, who've been engaged since late 2010 but took time along the way to welcome a son and daughter, are rumored to be getting married over the Fourth of July weekend.
"If we are not going to be at someone's house hanging out playing beer pong, this is the closest thing to it," bridesmaid and longtime friend Stephanie Terblanch told People about the co-ed party, noting that they're all such good pals nobody wanted to split up the group into his-and-hers events.
Another source told the magazine there was nothing X-rated about the evening: "It wasn't about being naughty."
Somehow, as this will be Simpson's second marriage and she's already a mother of two, that's comforting.