Hello Anna, I believe you'll find the following support article that I linked below helpful in understanding how to manage turning webcams on & off. Please note that for GoToWebinar specifically, only staff members (aka: Organizers & Panelists) are allowed to share webcam.
If someone has joined in as an Attendee and need to share their webcam, you will have to promote them to a Panelist (or Organizer) first. If you wish to promote someone while in session, find their name on the attendee list, right-click on their name, and you should see the option to 'Make Panelist'.
Share Your Webcam (Desktop app)