friendship society; fan club; supporters' association
A named group or membership organization for supporters, fans, or like-minded people. Often appears in proper names such as 〇〇友の会.
This museum has a membership organization called the 'Friends of the Museum'.
彼女はアイドルの友の会に入っている。
She belongs to the idol's fan club.
Compound of 友 (とも, 'friend') + の (possessive particle) + 会 (かい, 'meeting, association'). The exact historical derivation is uncertain, but the pattern is a common way to form names of societies and clubs.