Doorkeeper is an event promotion service that helps promoters build a network of participants called a "community." Invites for future events can be sent instantly to everyone in a community, and upcoming events automatically appear on community members' calendars. Should a promoter decide to switch services, they can easily export community members' information and take it with them.
When an event is created, Doorkeeper can share its information with other event listing sites. Instead of just getting a count of people who say they're coming, promoters can choose to charge in advance for events and then check participants in using a ticket scanning app. They can also send reminders to people who signed up for an event in advance to make sure they turn up.
The Doorkeeper API allows users to retrieve all of the events listed on Doorkeeper or the events for a given community. The Doorkeeper website and API documentation are offered in both English and Japanese.