In Word Line, you can invite a friend to earn 150 in-game coins per friend. Here's how to do it:

1. Open Word Line.
2. In the main menu, tap on the Settings icon in the top-left corner.
3. Tap "To invite a friend".
4. Choose whether you want to invite your friend via SMS or a messenger app.
5. Send the invite link to your friend.

Once your friend installs Word Line using your link and reaches the 12th level in the game, you will see a pop-up in the game telling you how many coins you have received.