Invitation System will only work when you disable the registration system and enable the Invitation System from admin panel
also you and only you (admin) will be able to generate Invitation codes from admin panel and send them to the users you want via any mean like Facebook or whatever (direct connection).
Also Sngine give you the ability to Send the Invitation codes via email from admin panel.
After that user will get the invitation email
To use it in the registration as you can see here:
That is it 😉