Start by setting up a Waitlist Form for an event. We suggest that you setup a new one for each major event in order to keep track of who's intested in which event. In addition, it allows you to send an e-mail to everyone on the waitlist when tickets become available.
Keep in mind, when you're done with your first form, copy it to create the next.
When setting up the event chekc the option that says When tickets sell out, allow people to join a Waitlist. This will then display the ability to assign a form.
When your event sells out (or if reserved and all tickets are locked up), your members will see the option to Join the Waitlist which will take them to the form.
Now when tickets become available, you can send a newsletter direct to everyone who signed up on the waitlist form.
Version 2.0 with automatic notifications, reserving inventory, etc. are in development.