To mute someone else on the call (we call this feature “remote mute”) just click or tap the microphone icon (for attendees joined on Biba) or phone icon (for attendees dialed in on the phone) next to their name on the roster. You can tell if someone is muted because the mic or phone icon will have aline through it.
The other person will get a message that they have been muted by you if they're on Biba, or a "you've been muted" message if they're on the phone.
Keep in mind that you can't remote UNmute someone, only they can do that. On Biba, the muted user can tap or click the muted icon to unmute, and on the phone, dial *7.
The organizer can also enable Event Mode from the More menu. This will mute all present attendees, and anyone joining the call will join in a muted state. You can read more about Event Mode here.