Clubs was a feature that was once available on Duolingo's mobile application. It was created for the purpose of learners being able to "connect online and stay motivated"[1]. This was accomplished by joining a small group that competes for the highest XP score, while also encouraging one another with an emoji-based chat system.
The limit for a club group was fifty members. Any user could make their own club or join someone else's. You would join someone's club by typing in a six-digit code provided by the club admin, or by searching through a list of open clubs. The club admin made decisions regarding who can join and whether they can accept new applications to the club or choose an avatar for the club[2]. You could join a club for each language that you were taking.
The arrival of the Leaderboards feature led to the discontinuation of Clubs.