Only lets users with an invite code register for a WordPress site.

wp-registration-keys.php

<?php
/*
Plugin Name: WP Invite Codes
Plugin URI: http://pmg.co/
Description: Makes wordpress an invite only community.
Version: n/a
Author: Christopher Davis
Author URI: http://pmg.co/people/chris
*/

add_action( 'init', 'wpse15535_add_shortcode' );
function wpse15535_add_shortcode()
{
	add_shortcode( 'invite-form', 'wpse15535_invite_form' );
}


function wpse15535_invite_form()
{
	?>
	<form action="" method="post">
		<input type="hidden" name="wpse15535_action" value="send_invite" />
		<input type="text" name="wpse15535_email" />
		<input type="submit" value="<?php _e( 'Send Invite' ); ?>" />
	</form>
	<?php
	// Do some stuff if our action is set.
	if( isset( $_POST['wpse15535_action'] ) && 'send_invite' == $_POST['wpse15535_action'] )
	{
		// Get the email
		$email = isset( $_POST['wpse15535_email'] ) && is_email( $_POST['wpse15535_email'] ) ? $_POST['wpse15535_email'] : false;
		if( ! $email ) return; // bad email? bail.
		
		// generate a random 30 character string.
		$key = wp_generate_password( 30 );
		
		// store our keys in the options table
		$opts = get_option( 'wpse15535_keys' );
		$opts = (array) $opts;
		$opts[$email] = $key;
		update_option( 'wpse15535_keys', $opts );
		
		// Send an email!
		$message = "You're invited to join a sweet community! Click here: " . home_url( 'wp-login.php?action=register&invite_key=' . $key );
		wp_mail( $email, "You're invited!", $message );
	}
}

add_filter( 'registration_errors', 'wpse15535_register_post', 10, 3 );
function wpse15535_register_post( $errors, $login, $email )
{
	// bail if something has gone wrong already as they won't be able to register.
	if( $errors->get_error_codes() ) return $errors;
	
	// If our key isn't set an error and bail
	if( ! isset( $_REQUEST['invite_key'] ) )
	{		
		$errors->add( 'no_key', __( 'You need an invite code to register for this site' ) );
		return $errors;
	}
	
	$opts = get_option( 'wpse15535_keys' );
	
	// see if this email has a key and if it matches what was submitted.
	if( ! isset( $opts[$email] ) || $_REQUEST['invite_key'] != $opts[$email] )
	{
		$errors->add( 'invalid_key', __( 'Invalid Registration Key' ) );
		return $errors;
	}
	
	// everything okay? Just return the errors and let it go through.
	return $errors;
}

add_action( 'register_form', 'wpse15535_add_key' );
function wpse15535_add_key()
{	
	if( isset( $_GET['invite_key'] ) )
	{
		echo '<input type="hidden" name="invite_key" value="' . esc_attr( $_GET['invite_key'] ) . '" />';
	}
}