The profile picture of every participant in a meeting is displayed in the Participants section of the meeting dashboard. There are participant controls beside each participant's profile picture. One of these controls is to raise/lower their hand, and another is to mute/unmute. If you, the moderator, mute everyone on the call, then they would have to notify you if they would like to speak. They can do this by "raising their hand". Once you see their hand raised, click their unmute button and they will get a notification that they have been unmuted. you may then "lower their hand" or they may do it themselves.