The Non-Participant List shows the scouts and adults who have indicated they do not plan to attend the event.

You'll find the button that produces this report on the detail page when you select an event from Maintain Events.

Click the Print Non-Participant List button to produce the report.  This button may also be found at the bottom of the Troop Calendar detail page.

The report is sorted by patrol, showing each scout and each adult who has indicated they do not plan to attend.