allowing to define default values via the GET or POST

I plan to extend a existing project with user logins. For this, I was trying to define default values for the add and register forms. The admin has a login and will be able to create accounts for other people by adding them. My intention is to suggest the already known email address of the new user by using a GET parameter. The URL will be something like www.yourdomain.com/member/add?Email=test@test.com

I thought this might be useful for others as well.