On Saturday, the pair attended a cocktail party hosted by Biel's stylist, Estee Stanley, at her Los Angeles home. And the soiree drew out a bevy of stars: among the 100 guests were Ellen DeGeneres, Amy Adams, Lance Bass, JC Chasez and Timbaland. Biel's parents, Kimberly and Jonathan, were also there for the celebration.
As for the couple, a source tells PEOPLE, "Justin and Jessica arrived at the party looking very excited. Jessica was stunning in a long dress with her hair down."
"Estee created a very warm, welcoming and festive party for the couple," adds the source.
After some live music, a deejay took over and guests danced to popular hits, including Madonna's "Holiday," Michael Jackson's "Good Times," Whitney Houston's "I Wanna Dance with Somebody" and Usher's "OMG."
Justin and Jessica "danced and looked very happy," the source says.
Afterward, there were speeches, and the source says Timberlake seemed really "touched" by Timbaland's remarks. "You could tell that they have a very special friendship," the source says.
The couple left the party at 12:30 a.m. and returned to their Hollywood Hills home.