The couple, who will wed in the L.A. area on Saturday, and 75 of their friends and family celebrated on Wednesday night at the newly opened Dakota Lounge in Santa Monica.
The singer, a.k.a. Stacy Ferguson, 33, and Duhamel, 36, were greeted with cheers from their guests – including Fergie’s Black Eyed Peas bandmate, will.i.am – when they arrived at their candlelit party just after 9 p.m.
Partygoers dined on Kobe beef sliders, tuna tartare and shrimp skewers, sipped champagne and danced to a deejayed mix of ’80s songs and newer hits.
“Josh and Fergie usually spend so much time away from each other, so they wanted to make this week special and spend as much time together as possible,” says a source.
“So they decided to have a joint bachelor and bachelorette party and have their friends mingle and get to know each other before the big day.”
Fergie, who wore a short, sequined black dress, “is having a blast,” the source told PEOPLE during the party, which lasted past 1:30 a.m.
• Reporting by PERNILLA CEDENHEIM