Invitation System in Socio
Invitation System will only work when you disable the registration system and enable the Invitation System from admin panel of Socio.
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, Whatsapp or whatever (direct connection).
Also Socio give you the ability to Send the Invitation codes via email from admin panel.
After that user will get the invitation email and user can register to your website using code.