Re: invitation to GS
Every thread has a “THREAD TOOLS” menu on its TOP RIGHT side, there is a link in it:
As for referring and invitations well, word of mouth is the king.
As far implementing this feature site-wide rather than thread based then there are plenty of scripts for vbulletin as well as “refer a friend” standalone scripts that can be installed to allow users to send emails to their friends and family telling them about gupshup forums!