In the event dashboard, click on "Registrations".

From the sidebar, select “Setup Mailer” under Registration.

You can click on "Manage Mailer" to update the content of the mailer and make the changes according to your preference. 

Click on the "Edit" option to update the details of the mailer like Name, Subject and Send to fields.

Here you can fill out the details and click on the "Save" button. 

In the “Available To” field, select “Group”, and then, in the “Select Groups”’ field, chooses one or more groups, then click save.

If you scope the registration confirmation mailer to a set of groups, the registration confirmation will only be sent to attendees who belong to any of the selected groups.

How to create reminder mailers that can be sent to registered attendees?