This article will show you how to create an invitation only event (i.e. participants can only register if they are invited to the event).
Follow the standard steps when creating a New Event Registration. When filling out the form, you will see the option to set the maximum number of individual participants.
When filling out the registration setup form, set Maximum Individual Participants to "Invite only".
Agree to the Terms & Conditions at the end of the page to finish creating your registration.