Activity Detail (w/Absentees) Report
This report is exactly like the Activity Detail report, but it also includes a section showing all members in the database who did not attend the activity.
The "absentee" portion can be quite extensive, so running this report is not recommended for large organizations. i.e., if the entire membership typically attends the activity you run the report for (via Criteria), you should be fine. But if you have 100,000 members and only 50 attend the activity, you don't want a list of 99,950 absentees, because the report will be hundreds or thousands of pages!
(If you have a large organization, use the normal Activity Detail report for attendees, and then run a separate membership report like the Contact List, and use the "Absent from Activity" member criteria to only select members who didn't attend the activity in question.)