If your event is an invite-only event, you can choose Invite-Only event option with a password protected for your events registration under Settings section when you create an event. If this option is checked, your attendee will be asked to enter the invitation code to register the event. 

Here is an example screenshot of how your attendees are asked to enter the invitation code to register the event.

Private Event Option

 
Last updated: 31-Mar-2016