I am developing a complex site with 2 distinct user roles one of which is called 'client'. Each role has a unique registration page.

Presently, the link in the email invitation looks like the following:
http://testsite.com/og/subscribe/4

but, I wonder if you guys can tell me how I might amend that to hold some additional data like the following:
http://testsite.com/og/subscribe/4/client

I know the variable to edit is !group_url but where do I amend it or create a new customized variable?