strange requests to join

I host a monthly neighborhood book club meeting; members can only join through the email link, which changes every month.    Almost every month, a day or so after the meeting, I get an email saying "xyz is waiting to join your meeting" - what is going on?  Once in a while, I  recognize "xyz", and figure that it's a person on my list who accidentally hit the link again - but who are these people I don't recognize, and why doesn't the link expire after the meeting time is over?