The Saturday party took place at the home of Timberlake's longtime friend and HomeMint partner Estee Stanley, People reported.
"Jessica and Justin were mingling both separately and together throughout the party," an attendee told US Weekly. "Jessica seems to be very close with Justin's friends and vice versa. You can see that they have been dating for a long time and really know each others friends and family well. The party was a fun celebration and a preview of what's to come with the wedding."
Timberlake and Biel, who became engaged in December, are set to get married later this year, US Weekly reported.
Earlier this month, the pair were seen at jewelry boutique Eli Halili in New York City's Soho neighborhood, shopping for wedding bands.
Although the couple didn't select wedding rings at that time, they did leave the store with a $2,200 ring and and two necklaces.