private cheer group; unofficial cheering squad
Refers to a cheer group that is not officially recognized by the school or organization, often formed by fans or alumni.
There are several private cheer groups at that university.
応援団 is a general cheer group, while 私設応援団 specifically means a privately organized, unofficial one.
Compound of 私設 (privately established) and 応援団 (cheer group).