It has a few different meanings:
1.) A group of musicians and actors during the 50s and 60s that were very social, decadent or did expensive, glamorous and fashionable things and partied a lot. They include , Dean Martin, Sammy Davis Jr., actor Peter Lawford, and Comedian Joey Bishop.
2.) [jokingly derogatory] colloquial phrase to refer to a group of friends, or a clique, that excludes other people and do glamorous-appearing things and think they are wonderful. It means something like "a group of teenagers who are very close to each other and think they are the best/most interesting group in the world; maybe sometimes annoying and always going out to parties and having fun."