1. First, go to the Registrants tab.
  2. Click on the green downward arrows next to each registrant that did not attend.
  3. Select Mark As Will Not Attend.
  4. Repeat this step for every person that did not attend.
  5. Then to the left of the event under Tasks, click Update Event Attendance.
  6. Mark Attended for the option to "update all registrants whose attendance is unspecified to."
  7. Click Save.