Doujin Circle (同人) is a group of people who share a common job, are a family, share interests, hobbies, fandom, or another kind of an activity or environment.
The term derives from the Japanese dōjin that can be translated as clique, fandom, coterie, society, or circle. That word is romanized as doujin and used with the word circle, so that it's not confused with doujinshi.
The term appears especially in anime or manga that rely on relationships between characters or Japanese-like tradion, such as Historical, Fantasy and Slice of Life works. It can be used by the otaku in the context of sharing one fandom.