Query regarding Registration Required

I have a series of meetings set up in Zoom. I circulate the calendar feed URL that Zoom provides so that all of my potential meeting participants can subscribe to the feed and get the meetings, and the Zoom link for the meetings, into their calendar application.

At the same time I want to know who is participating in each meeting. The best way as far as a I can see is to set the 'Registration' option to 'Required' for the meetings.

In this way I get a record to show who registered for each meeting.

Here's my question: If someone has the Zoom link for a given meeting (i.e. via the calendar feed), can they join the meeting without registering? If they can't join, what will they see when trying to join the meeting?