Moderators as well as meeting hosts both can change participants’ roles. To Mute All participants host or moderator need to press the "Mute All" button:

Then follow the further instructions on the popup that follows. Pick the devices you wish to mute.
Don't allow participants to turn on devices - slide the switch to the left.
Press the "Apply" button to confirm.

To send a request for enabling the user's microphone or camera, press the camera and microphone buttons where highlighted in the screenshot.
 
In the pop-up window, click the "Confirm" button to send the request.

The user will get a popup with the request. The participant will get a choice to confirm or decline the request.