a community that shares a mythology in Jeju Island

신화 향유 집단 sounds strange to me. 향유 or 향유하다 is a formal phrase used about rights, freedom, or privileges. It means to have something as a useful privilege or resource, and can be translated as "to enjoy" in the sense of availing oneself of the thing. It sounds strange with 신화, though, because 신화 is not something people appreciate or find useful. So it's hard to find a sensible phrase for this weird sounding original. One possibility is something like "People (of the Jeju island) who share the myth/mythology", meaning the group of people who know and/or care about the myth as distinguished from outsiders who don't.