probably a pun on the "The Rat Pack"

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."

Yes, it's a play on words of "The Rat Pack". The film is about a group of famous snooker players from the 1980's - "rack" is a term used in the game of snooker, hence the pun. I believe some of them had a reputation for partying/drinking hard, similarly to the original rat pack, as defined by Katherine.