The current Presenter can invite any attendee to start their web cam, provided the attendee has enabled their camera. A grey camera to the right of the mic icon lets you know that user's web cam is enabled, not on, but just accessible.
An attendee cannot start their web cam until the Host and/or current Presenter allows them access.
To allow access, click the gray web cam icon.
When the attendee starts their web cam, their icon will turn green.