For each person you invite, you receive a different amount of playing dice to advance in the game.
On sign-up, the referred gets 100 Poké Balls and perhaps some other stuff to start their adventure with. The referrer then gets rewards based on activity of the new user in the game - you get an incubator for example, when the new user evolved 100 pokemon.
Whenever you invite a friend, you receive 9 NCG immediately. When a friend achieves LV20, both you and your friend receive 9 NCG additionally.