Sandy’s story in Grease is a classic one in pop culture, but it doesn’t make sense until the movie musical’s prequel, Rise Of The Pink Ladies, provided some much-needed context. Sandy’s story is told in 1978’s Grease, but before Grease comes its prequel, which tells the origin story of one of Grease’s main greaser groups, the Pink Ladies. The Pink Ladies struggle to carve out a spot for themselves at Rydell High, but eventually they endeavor to fight for a sense of belonging for all students, especially new kids at Rydell like Sandy.

https://screenrant.com/why-sandy-joined-pink-ladies-grease-movie/