This cannot be done with the built-in functionality of calendar events, but a work-around is to use a survey and security settings.
  1. Give the event a special security category so that only people in that category can view the event
  2. Create a survey with the questions you would have included in the waiver
  3. When you go to edit the survey, under step 2 "Include Questions" click on "Create Group by Survey Response"
  4. Create a group with the security category set up in step 1 and identify the group in the survey
  5. Create a filter so that only when questions are answered a certain way is someone added to the group