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An invitation system (or invite system) is one method of website registration. Unlike open registration, where all users can join, and closed registration, where there is a moratorium on new users joining, invites allow current users to select who can join, while allowing site administrators to maintain a stronghold on the population of the service.
Existing users receive a set number of invitations (sometimes called "invite tokens" or "invite codes") to allow others to join the service. Those invited are either sent a specialized URL, or have to enter a single-use code.
Invitation systems are usually temporary. They are typically used for services in private beta testing, in order to control the number of users on the service. In other cases, they can be used due to limited availability of server resources. There are a growing number of sites which use invitation systems on a permanent basis to create exclusivity and to control quality of user-generated content. Rarely, they may be used on a permanent basis in order to aggregate social network statistics (all users will ultimately have a traceable connection to all others). They can additionally be used to avoid trolling or spammers (all users trust all others), which is usually a positive side effect of the invitation system, even if it was unintentional.
Sometimes, a tiered invitation system may be in place, wherein those higher up on the pyramid will receive copious amounts of invitations to hand out, whereas fourth- or fifth-generation invitees (of which there are a lot more, potentially) may receive few or none.
Some prominent services which were once invitation-based include e-mail provider Gmail, social networking service Orkut and journal/blog site LiveJournal. Oink's Pink Palace, a BitTorrent tracker, was invitation only until its forced shutdown in October 2007. Google Wave is currently in an invitation-only stage.