One of the biggest factors in persuasion is the concept of exclusivity. If people feel like they are getting in on something that not everyone else can get in o

One of the biggest factors in persuasion is the concept of exclusivity. If people feel like they are getting in on something that not everyone else can get in on, they're more likely to do it. Be generous with the amount of invites so you can grow. If you like your current community of users, keeping the system invite-only will generally keep the community at that high caliber.