1. Log in as administrator
2. Either add a new event or edit an existing one
3. Set a free registration pricing by checking the Enable Registration option.
4. Once the registration options are displayed set an attendance Limit (0 means unlimited) and uncheck the Payment required option:
5. In the Pricing(s) options for the registration, set Payment required to No and Price to 0.0. Finally set the start and end time when RSVP should be enabled: