In the event dashboard, click on "Registrations".

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

This will take you to a screen with a list of mailers. You’ll also notice that it is in the “Draft” state, which means this email has not yet been sent. \
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Next, click on the edit button against the mailer that you want to be sent only to a specific group.

Choose the relevant groups under "Select Groups". 

Now, when you “Activate” the mailer, it will only be sent to attendees if they belong to any of the groups selected.

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 scope a mailer to be sent only to a subset of the registered attendees?