Have you ever seen social networks with an invitation system? Many use this approach to build buzz around their community and even control who can join their network. WP-Invites happens to be a free plugin that lets you add such a feature to your website. It lets you generate invitation codes for your community and use them to make sure only a certain number of people can join your community.
The plugin is compatible with WordPress, WordPress MU, and BuddyPress. It lets you make sure only people who have invitation codes can register for an account on your website. The plugin is available for free. It may not be the right choice for every site, but it does give you more control over who can join your community.