We rely heavily on the registrant's confirmation number for many of our tasks, research, verifications, and finance needs.
Perhaps it would be a viable option for you to specify ahead of time that the registrants will receive a unique group confirmation code that will be used when you run a "thank you" survey afterward. If they were surveyed and asked to input their group's confirmation code then every group who completed the survey could then get a follow-up email. In the follow-up email, the group leader would be asked to submit a screenshot of their event attendance and a digital confirmation (check box & "signature" of the group leader).Maybe that would give your event something beyond the honor system. Even some of the Cvent Celebrity challenges ask for screenshots for confirmation.The screenshot could be mentioned at several way-points during the meeting. I would not suggest mentioning it in the housekeeping items at the beginning of the meeting lest people screenshot and "walk out". Intermingle that message.
Still honor system-esque, but more evidentiary. I think it would also depend on the skill level of your attendees.