In the Admin UI tool, on the sidebar select the Appointments List page (not the Appointments page, on the Event Management level). Check the box ‘All Appointments’ and select the roster event from the Event drop-down menu, and click the Search button. 


On this page you can email participants or delete appointments(registrations).


On the top of the page is the Send Email button, you can then select the option to Send to All or to Send to Selected. A dialog box will come up that looks like this: 

This will let you send a Reminder/Follow-up email or a Custom email to your group of participants. You can use the current email template or you can customize as needed. The template for both emails is located on the Settings / Notification Setup page.