Anyone with management rights of a webinar can add or register an attendee before or after the webinar. If you register someone for an upcoming webinar, they will receive a confirmation and subsequent event reminder emails. If you register someone after the live event (to access the webinar recording) they will not receive an email confirmation.
To add or register someone for a webinar, you must have admin privileges. Go to the event Promote & Manage page. Then Manage Event Registrations. Then click the Add Registrants button. Then type or upload the email address(es) of the person you'd like to add. You can add individuals or upload a CSV file of multiple people.
Note: It is your sole responsibility to inform ANYONE you add to any registration list.