It is correct to not capitalize greasers, as the term does not refer to one specific gang. The book names several greaser gangs, such as the one that the main characters belong to and Tim Shepard's gang. Capitalizing implies that the main characters are the only greaser gang, instead of it being a general reference to poor, sometimes juvenile delinquent, youth.
Start a discussion with MargaretMary23
Talk pages are where people discuss how to make content on Wikipedia the best that it can be. Start a new discussion to connect and collaborate with MargaretMary23. What you say here will be public for others to see.