Thanks for replying.
Yes, all attendees will be enrolled on the Moodle course, 2000+ in subject area groups.
So far I've got the idea that the Moodle course would have all of these Quizzes (compulsory sessions) or Schedulers (self-sign up) on them which would have group restrictions on them, and we give students the link to the quiz in the session, they have to complete it in that session and then awarded a badge once completed. For self-sign up the staff member running the session would give the student a grade if they attended the session to be allocated a badge.
It's just the scale of this project, which now seems incredibly daunting!