Other celebrity guests included Nicole Richie and her husband Joel Madden and actress Alyson Hannigan.
Obama is spending two days in Hollywood to raise funds for President Barack Obama's re-election campaign, and families of four spent $2,500 to attend the party, which included face painting and balloon sculptures.
The First Lady told guests, "Barack can't do it alone. He's not Spider-Man. He's not a superhero. He's a human, so we need your help. I am not just talking to the adults here today. I am talking to the young people here as well.
"All of our young people - you might not be old enough to vote. You vote at school, I know - I met several young people who are going to be voting for my husband... We accept those votes. You can play an important role in this election, too. I want you all to feel empowered."
Stefani's No Doubt bandmates, drummer Adrian Young and guitarist Tom Dumont tweeted about the event. Young wrote: "Life in (No Doubt) land gets more strange and wonderful everyday," while Dumont called Obama "so warm and sincere".