For the Course of interest, do the following:
  1. Click on the Blackboard Collaborate Ultra tab
  2. After the page has loaded, select Filter by "All Previous Sessions"
  3. For a given session, select the menu (circle with the 3 dots inside), then select View Reports
  4. For the date of interest, select View report.
  5. You should now see each attendee for that given session, also when they had joined or left the session.