Sending a message to all registrants for an upcoming meeting

You cannot send a generic message to all registrants through Zoom directly. What you can do through Zoom is edit the confirmation email with a bit of added info and then resend the confirmation email to all your registrants. This article goes through how to resend the confirmation emails: https://support.zoom.us/hc/en-us/articles/360054446052-Managing-meeting-and-webinar-registration

To edit the confirmation email, you just need to switch to the Branding tab and edit that email template. Keep in mind, you can only add a bit to the email, not edit it entirely. 

Beyond that, you will just need download the registration report, take the emails from that, and use your own email service to send them all an email. 

We are able to send to all registrants of an upcoming meeting at once using salepager.com