(Business 2.0) – Dying to know what was said about you at the last slumber party? Time was when such curiosity ended in junior high, but now big-name brands are trying to get mentioned in girls' gossip. To put products in the hands of "tween" influencers--trendsetting girls ages 8 to 12--companies such as Capitol Records, Disney, and Hasbro are hosting marketing events modeled after sleepovers. All are clients of Girls Intelligence Agency, a Los Angeles market research firm that distributes slumber-party kits--packed with freebies and ideas for themed games and snacks--to 40,000 girls willing to introduce friends to new products. "If you don't have word of mouth as part of your marketing campaign, forget about it," says GIA founder Laura Groppe.

Disney used the kits in January to launch an animated TV show called W.I.T.C.H. GIA asked 500 hosts to invite 10 to 12 friends each to watch a few episodes and fill out surveys; the show was an instant hit with that demographic. Last year Capitol Records tapped GIA to invite 6,000 girls to a series of parties promoting a teen artist named Skye Sweetnam. Attendees watched a music video and critiqued the singer's hair and clothing, and Capitol responded by making her image more conservative.

Also getting into the game is House Party, a startup based in Irvington, N.Y. When the company organized 1,500 parties this year previewing a new Nickelodeon Networks show, 70 percent of attendees told five or more people about it. For marketers, that's the juiciest gossip around. -- JENNIFER ALSEVER