It's quite rare for people to add users themselves. It's not rocket science to sign up yourself. Their
Support Toolkit has an option to make new users though. Also if you have a 'club' or something like that with a closed membership you can use the antispam countermeasures Q&A and set up a question telling them to enter the code they've been sent. That way you can make an entry code for the registration page that only the people you email will know and therefore only they can register.