New users registration: visitors can create accounts, but special roles only by invitation

I think it would be useful if Invite module would give us opportunity to check in "User settings" additional tab: visitors can create accounts but specific roles only by invitations.
I mean that if site have some roles - one is public and one is for specific users - it is useful to make possible different registration: for usual visitors (and they will gain public role) and for special visitors who came by invitation (and they will gain special role)