To the right of the video window, there is a list of all participants that you have invited to this session. If someone is in the waiting room, you will see their name. Click their name and they will auto-join the session.
Here you can see Harry Potter has joined the waiting room but Carol Park has not yet joined this session. I can let Harry in to the session and begin the session while Carol joins.
In the image below, I've now let Harry into the session and Carol still has not joined. When she joins, her status will change to Waiting Room - Click to join.