FTP to your site and goto
modules > mod_login > tmpl >
EDIT
Default.php
Remove or comment out.
<li>
<a href="<?php echo JRoute::_('index.php?option=com_users&view=registration'); ?>">
<?php echo JText::_('MOD_LOGIN_REGISTER'); ?></a>
</li>
and it will take the registration URL out of the login box.
Email the people you want to register a link
http://www.*************.com/index.php?option=com_users&view=registration
You might want to make sure that admin verifies the registration to activate them so you can keep an eye on the lin being used by only the people you want it used by...
Adam.