[support] modify the og invitation process!

[Aldo Martinez Selleras]

well, i'm doing a invitation response system for OG, when the user 
recieve the invitation,  this go with a url to go to the site, well if 
user have a user account in my site, i can send to the login form with 
the parameter "&destination=", i can do this for register form too; but 
i need when the user is registering for joining a group, after the 
register go to og/subscribe/$gid page, but for this i need send the 
warning message from successfully register with the URL 
user/register&destination=node/$gid

how can i do this happens only when the user registering for join a group?