To mute attendees individually, mouse over their mic or telephone icon next to their name:
To mute everyone that is currently in your meeting, click the icon at the top of the Attendee area called Mute All:
To have all subsequent attendees join muted, tick the box highlighted in red above.
You may also disable the chimes heard when someone comes in or exits by ticking that box:
To collapse the left panel from view, just click the icon:
This view is managed by each attendee in a Meeting for their personal viewing space.