On the event page, in the tab Applications, press the + button. (Currently you need to hold CTRL while clicking the button to open the registration form in a new tab. This will be the default soon.)
The registration is automatically connected to the selected persons and profile fields do not need to be synchronized. You cannot change the persons primary email address in the registration form. If you need to change the persons email address, first update the primary email adress on the person’s profile page.
Check the relevant checkbox if you want to send a confirmation email. To send a confirmation email later, in the list of applications, select the applicants you want to send the confirmation email to, and from the Action button ( ), select Send confirmation email.