An invitation system is a method of encouraging people to join 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 existing members. The basis of the invitation system is that a member can grant approval to a new user without having to consult any other members.

Existing members may receive a set number of invitations (sometimes in the form of tokens) to allow others to join the service. Those invited to a website are typically sent either a specialized URL or a single-use pass code.