The Waiting Room can be turned on from the Attendee panel. Click on the Attendees button in the right side of the top toolbar.
You'll see near the top of the panel the Waiting Room, and it will say either ON or OFF. Just click on the ON/OFF button to toggle the waiting room.
⚠ Note: Disabling the Waiting Room while guests are inside waiting will instantly allow all of them into the event. This is a very effective way to begin larger events where lots of people are waiting to watch.