Within the Sessions tab of the app, you will see Open Gym at the top right of the screen.  

When there is a RED dot that means Open Gym is turned OFF and is not collecting heart rate data.

When there is a GREEN dot that means Open Gym is turned ON and is collecting heart rate data of anyone in your coach app who is within range with a paired heart rate monitor.

Tap the OPEN GYM button. 

Here you will see a START button if entering this screen while Open Gym is OFF.  

To officially turn ON Open Gym, press the START button in the upper right corner.

If no members of your platform are within range then the iPad screen will be empty until someone comes into range.  You will see the Time Elapsed count upward.

While the iPad screen remains empty (no members within range), the connected/mirrored TV screen will show the Morpheus Coach moving screensaver as shown below. 
To ensure this is the case, go to the PROFILE tab of the app and scroll down until you see DEMO MODE.  Ensure that is toggled OFF.  
When Demo Mode is toggled ON, a mirrored TV screen will show exactly what is on the iPad screen.

As soon as one or more of your members comes into range of the hub, they will automatically appear on the iPad screen and the TV screen as shown below and their workout will begin recording.

The iPad will also show any other "unknown" ANT+ heart rate monitors with the ID number of the device.  It's unknown because it is not paired to any member of your coach app platform.  In an upcoming version of the app, these will be clickable and will allow you to assign that device to a member.

TV screen visual shown below.  The TV will never show unknown devices as shown above on the iPad view.

As mentioned in this articlewhen Open Gym is ON, you can still navigate the app while heart rate data continues to be collected in the background.  To do this, just tap the back arrow on the upper left corner.

When you navigate back to the SESSIONS tab of the app, you will see that the OPEN GYM button has a green dot next to it.  This means that Open Gym is turned ON.  And you can tap that button to get back into Open Gym as needed.

If you want to stop Open Gym, you can do so by pressing the STOP button in the upper right corner.  If there was anyone within range whose workout was being recorded, this will end their workout and they will receive the post-workout email.