Could you elaborate? Email invitations are often controversial, since they may be considered spam and may get your mail server blacklisted. Bad karma for your site. For example, services like mailchimp won't let you use emails that haven't explicitely subscribed or registered to your service already.
Right now there is no easy way to add a user or invite a user to the site.
Let me explain my usage:
I use the site for tutorial, you can see it at CCCure.Training
Sometimes a company may buy accounts for 10 students. I need to invite to ten users to the quiz and it is not easy to do this.
Most portal would allow to import a list of emails from an XLS file to add a group of users to the site.
Best regards
Clement