Joseph will contribute to the site's online-only magazine with a blog, style tips, video interviews with notable fashion designers, as well as engaging in live chats with Stardoll's community.
Stardoll attracts more than 8m users around the world per month, 94% of whom are teen or tween girls, with its mix of outfit creation, celebrity and social networking. Its UK sales house is Unamis.
Mattias Miksche, chief executive of Stardoll, said: "Stardoll's partnership with Elle legitimizes Stardoll as an ideal platform to communicate with young fashion-oriented woman."
He added Stardoll wanted to help Elle get its offline content to its community.
Joseph said: "I'm looking forward to interacting with this enthusiastic group of fashionistas in so many fun and exciting ways."
Stardoll is backed by two well-known internet venture capital investors, Sequoia Capital, which has a track record including Google and Yahoo!, and Index Ventures, which put money into Skype.