How to gameshare between two PlayStation 4 consoles with Sony's 'Share Play' feature

You can gameshare on your PS4 to let a friend on a different PS4 console play a game in your game library, even if they don't own it.
Gamesharing, known as "Share Play" for PlayStation consoles, is a convenient way to play a game with another person even if only one of you owns the game.
To enable gameshare on your PS4, you need to sign into the other person's console, and change your account settings to set that console as your primary PS4. 
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