1) Go to Sites > [site name] > Email Notifications.
2) Click on the email notification you wish to customize.
3) Click "View"
4) Scroll down to the bottom of the page and click "Customize this message"
5) You can change the content via the text editor.
6) You can change the recipients by clicking on any of the options in the "Users" box and then clicking on the arrow pointing to the right. You will see that the user is added to the "Who receives this notification" box.
7) Click "Save Changes"