When someone else is on the page with you, the menu at the bottom left says so, and holds the session's controls.
The live menu rises at the bottom left of the screen while a live session is on. The state button carries a dot, green when the session is on and red when the connection is down, and the switches beside it each carry a count: people in the room, unread chat.
A press on the state button swaps the live menu for the session menu. What it holds depends on who you are: the host can invite, mute everyone and end the session, a guest can follow the host and raise a hand.
Share this page and enter the link, and the live menu comes up.