Baby-sitters Club was a series of books for teenage girls by Ann M Martin, popular in the 80s and 90s. They were popular with the same readers as Judy Blume and Paula Danzinger books.
The series was based around a group of middle school students living in the fictional town of Stoneybrook, Connecticut. They run a business called The Baby-sitters Club, where neighbours book the girls to babysit for their kids. The girls run the club on fundraising monies.
They key member were Kristy Thomas, Mary Anne Spier, Claudia Kishi, and Stacey McGill, all 13, but later on their younger sisters join the Bay-sitters Club, too. Best of all, the members are all best friends.