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?