If your friend wanted to invite students by email to join a learning group, I can recommend jegelstaff's excellent Registration Codes 2 module.

The module automatically adds users to a custom group when they enter a special code number on the sign-up page. You can include the code in an email link, or send it out by post.

There's some more info and a download link in this thread.

The current version prohibits members of the public from registering, but if you want controlled registration, it's ideal.

Thanks guys!