Each player has a unique code located in the Social tab under Settings. To use them, simply copy your code and send it to your friend.

To find your code, select (tap/click on) the 3 bars on the top right of the screen. This will open a pop-up window with your game options and settings.

From there, select the "Social" tab.

The code will be displayed as shown in the image above, and it can be automatically copied by tapping or clicking on the button directly.