After you have launched a session and have added everyone to it you will see their heart rate on the screen.  This is the pre-workout screen in the app.  

To start the actual workout, tap START WORKOUT in the upper right corner.

After pressing Start Workout you are transitioned to the screen that is shown below.

You will see the live heart rate and zone for each person and it will be color coded to the zone that they are in.

You can tap on an individual which will enlarge their tile and allow you to see more info about that individual.

When you are ready to end the workout, tap End Workout in the upper right corner.  

You will receive a prompt asking if you are sure you want to end the session.  

If you are, tap End.  If you want to resume the workout, tap Continue.

After tapping End, you will see a workout summary screen with the workout stats for each member of the session.

Check out the next article for important info after ending a session.