At the top of the featured page, you will see all of the upcoming live classes. If you see "Count Me In" this means the class has not started yet.
You will get a pop-up on screen and once you select "I want to Ride!" You can add the ride to your calendar and you will get a notification when the class is starting. If the class has started you will see "Join LIVE Class". Simply tap on the button and you will be joined into the live class.