When you are creating your event and specifying the Event Details, you can enter a number of Tickets Available and specify if you want to create a waitlist.
Checking the 'When sold out, allow people to waitlist for this event/eventlet' box and leaving the field blank will allow any number of people to waitlist for your event. This is very useful for still gathering information about potential attendees, even if your event fills up.
If all the tickets to your event are sold, then potential attendees will see the following message when trying to register for your event:
You will also see an additional option in your Administration views:
You can then choose to Invite those on your Waitlist: