How can I require guests to provide email address before joining a meeting?

There are a couple of ways of doing this: 

1. Require registration for your meeting, which will allow you to capture their email, as well as other info you may want. This article can show you how to schedule a meeting with registration: https://support.zoom.us/hc/en-us/articles/211579443-Scheduling-a-meeting-with-registration
2. Require authentication for your meeting, which means each participant must be signed-in to a Zoom account. This will not ask them for an email upon joining, rather it will require them to be signed-in to a Zoom account and the email associated with their account will be available in the attendee report afterwards. This article can show you how to require authentication: https://support.zoom.us/hc/en-us/articles/360060549492-Allowing-only-authenticated-users-in-meetings

Hope that helps and please make sure to mark the solution as accepted if this information is what you needed.

@Bort You aren't wrong, but it's missing an important clarification.

The email provided by a registrant upon registering is NOT connected to the participation activity (i.e. attending).

Meaning, the only way to relate the Registration event to the Participation event is by requiring authentication (i.e. the person must register with an email that is associated with an active Zoom account. This must be done before registration can be completed.)

The problem is that this causes friction and annoyance to registrants who do not have a Zoom account and do not want to set one up. The fact that you can't just require someone to enter their email address again when they click the join link (or better yet should be able to auto-detect it based on the email provided when they registered) and use that to tie Registration to Participation is curious to me.

Sources:

Zoom API email address display rules

Webinar Registration has been created API schema

Participant/Host joined webinar API schema