You get a bonus for inviting your friends and then they have to invite their friends.
Some games will give you a bonus when you invite your friends to play and link to them to your account. This bonus may be a one-time benefit, or it may be an ongoing benefit that improves the gaming experience for each friend that you add. This gives players a strong incentive to convince their friends to play. Those friends then have to sign up more friends and so on, leading to a pyramid scheme and viral growth for the game.
Often, this is combined with another Dark Pattern called Social Obligation where you and your friends can help each other in the game. This leads to a feeling of obligation and people keep playing the game even when they no longer enjoy it.