You don't need Master User for this. You just open the Joomla User Manager in your site admin, create an new user there. You just need to know their email address. Joomla should automatically send them an email with the username and password.
You only need to use Master User to log into the user account once it has been created. If you already know the password (which you will if you create the account yourself) you don't need the plugin. The plugin is really useful when you don't know the user's password, or you might have forgotten it, then it means you can use your administrator password.
