Open the Attendees panel by clicking on the Attendees button in the tool bar, the button is located fifth from the right in the right side grouping.
At the top of the actual attendee list you'll see the row called "Waiting Room" with a button that says either "OFF" or "ON".
Simply click on that OFF/ON button to toggle the waiting room on or off.
⚠ Important: Turning off the waiting room will allow everyone who's waiting into your Meetn room. This can be really helpful when you have a large event and want to let everyone in once you're ready.