Good way to root them out up front. Like self identifying targets.
Yep. We need attendance, too. And to follow them outside personally to talk with them, find out where they live, see how they arrived there, take some photos and videos, make it blatantly obvious that we see them. Every single meeting.
Vetting the meetings is a great idea. Third party holds meetings. Online signup with address and email address, get in with that ticket and ID.