Invite Only Sign Ups
Insecurity is an effective tool to attract new customers. Limiting access to a product or service to members triggers an emotional response –the fear of missing out is more effective than logic.
To trigger this feeling, you can restrict your product's access by allowing new users to join only when they are invited by a current users.
Startups like Gmail, Quora or Dribble used this tactic to create demand and virality.