Secure Invites is a WordPress plugin that allows you to only allow invited people to sign up.
For standard WordPress: just install the plugin as normal from the plugin repository.
For WordPress MU and WordPress MultiSite: there are two options. Either place the plugin in your /wp-content/mu-plugins/ directory (not /wp-content/plugins/), this method requires no activation. Or install the plugin as normal from the plugin repository and enable it for all sites in your network.
To enable the template form in your template page you should call the secure_invite_form() function like this:
<php? secure_invite_form(); ?>
There are three optional parameters in this function, they are:
So to set the CSS class of the form to 'inviteform', and the success message to 'Yay!' and the error message to 'Oops!' you would use this:
<php? secure_invite_form( 'inviteform', 'Yay!', 'Oops!' ); ?>
In addition, you can now automatically show the invitation form in your BuddyPress theme (if it is a child of the 1.2 or greater default theme, or it has all the same template hooks). Just tick the right boxes in the admin settings screen for where you want the invitation form to appear in your theme.