On each Event details page, there is a drop-down for sending an email. The drop-down contains a list of email templates that are available for the given event. When you select a template you are taken to the compose email page.  

On the compose email page there is a "To:" field that is used for entering the email address that the email should be sent to. The email autofill controls which emails are automatically populated in this field.  

For example, say you created a template that was designed for employees. On the email template configuration page in the "Email Autofill" setting you would place a check next to employees and nothing else. When the email is created from the template, the email address of all assigned employees will be put into the "To:" field.