Open a session from the schedule and tap Save my spot. That is it - your seat is held. Booking is included with your membership, and you can cancel any time from the same page.
Once you are booked, the page stays exactly as it was - the host, agenda and who else is coming stay in view. Only the button changes into You're in, with Open the lobby once the doors open (an hour before the start), Enter room while it is live, and Cancel booking underneath.
If a session is about to start or already live, the button says Join and takes you straight in (as long as there is room).