When you sign in on a mobile device it is automatically added to you account as a potential camera. All devices can be used as cameras or viewers and have those roles changed.
Cameras work like this:
-A device is added to you account when you sign into it.
-There is not need to “Add” it.
-The device you are using at the moment ( let’s call that device_1) can become a camera if you go to the bottom of the page and tap on “This camera”.
-When you sign in on another device ( let’s call that device_2) you should see device_1 as an available camera. You must sign in with the same account.
-Tap on that and it should display.
You never need to add a camera they, are added automatically when you sign in.
If you want to change which device is the camera and which is the viewer just switch which device is device_1 and which is device_2. Exiting the camera is attained by tapping the back button.
Here is an Overview of Presence: