It's fans of a band/artist who promote the band, their records and tours. This can be done in various ways and after the email that went out in the last 24 hours I'd expect to see quite a few new signatures on JT as that's one way the Bon Jovi street team members are being asked to help.

Other street team activities could be as simple as requesting said band's records on radio stations to putting up posters, word of mouth recommendations etc...

It's a way for fans to feel more part of the 'band' and to help, and for the band to get a stack of advertising!