An invitation system is a method of joining an organization, such as a club or a website. In regular society, it refers to any system whereby new members are chosen; they cannot simply apply. In relation to websites and other technology-related organisations, the term refers to a more specific situation whereby invitations are sent, but there is never any approval needed from other members. Popular alternatives to this specific version are open registration and closed registration. Open registration is where any user can freely join. Closed registration involves an existing member recommending a new member and approval is sought amongst the existing members.