How do I create an event that is 'invite only'?

When you edit your 'Event Settings' you can create invite only events using the 'Advanced' feature.

If you check the 'Enable Invite Only' box, your attendees will only be able to access the event microsite via a special link sent to them by email. Attendees you wish to be invited can be added to a cumulative list in the 'Invitations' area of your Event Dashboard. Once added to the list (either individually or imported using a CSV), you can send your invitation emails out and track their current progress - you can also re-send the emails to specific attendees if required.