It's on by default
Every room starts with the waiting room enabled. Nobody drops into your call unannounced.
The waiting room
You decide who comes in. The waiting room is on from the moment a room is created.
Guests wait
When someone opens your link or types the code, they land on a waiting screen with their display name until you let them in.
Hosts admit
As the host you see who's waiting and choose Admit for each person. You can also decline.
Share the link ahead of time
Because the waiting room holds people, it's safe to send the room link or 6-character code before you're ready to begin.
More on sharing a room in Share a room.