1.
club, circle (university/hobby group)
An informal group or club, especially at a university, organized around a shared interest or activity. Less formal and less competitive than a 部活 (official club).
Cultural note
In Japanese universities, サークル refers to casual student clubs, as opposed to 部活 (official athletic or cultural clubs), which are more structured, demanding, and competitive. サークル activities are more relaxed and social.
Usage
Sense 1 is the dominant meaning in everyday conversation. Common question at university: どのサークルに入ってる？ (What circle are you in?)