We've had an event this morning for which I've created a custom enrollment form with one field that is critical to us to collect the information (team affiliation) and report out
For some reason, the users did not have to go through enrollment after the time when the event had started. The ones who have attempted to join prior to start time did enroll.

We have the second session coming up today Could you please explain how to make the enrollment mandatory?

Hi, assuming you're on WBS26
You can add security to your event by requiring that attendees provide their registration IDs to join the event.

Attendees receive an invitation email message that includes a link that they can click to register for the event. Once you approve an attendee's registration request, the attendee receives a unique registration ID in a confirmation email message.

To require a registration ID for joining an event:

1. On the Schedule an Event page, at Registration, select Required.
In the Attendees & Registration section, select Yes at Registration ID required.