Dialpad Meetings allows a Participant to join a conference as a viewer — this Participant cannot speak during the conference or hear anyone else speaking. Participants normally do this if they're joining from a computer to view as a screen share but will dial-in from a phone to join to listen or speak.
Organizers and other Participants are able to see this type of viewer with the View Only label on their card. If you hover over it, you'll notice it says the Participant does not have any audio available.
In the list of all Participants, you'll notice again that there is no microphone next to the Participant's name to mute or unmute them.