Click the Mic button or the Mic+Cam button:
First, you need to make sure you have the attendee cameras and mics enabled within the Presenter Control Panel. The Mic and Camera buttons must be moved to the right, and green, in their on position.
Keep in mind, doing this only ENABLES the attendees to turn on their mic or camera, they still have to turn them on themselves. A host cannot turn on an attendees mic or camera.
Attendees must turn on their mics or cameras on. You have enabled them, but the attendee must turn them on.
Click Allow in the Browser dropdown:
Once they click the on button(s), they may have to click "allow access" located in a browser dropdown near the address bar. If the dropdown doesn't appear, they'll need to click the camera icon in the address bar. On Firefox it will appear to the left, on Chrome to the right.
For Firefox it will appear on the left side of the address bar like this:
For Google Chrome it will appear on the right side of the address bar like this: