How to access register or sign up page when applying ADFS?

Hello,

I have a WIF web application integrated with ADFS. It works fine to redirect ADFS when accessing the web page, then redirect back after authentication. But every time when accessing to web page, it automatically redirect to ADFS, which is not I really want for a new user without credential yet.

My scenario is: System Administrator will send an invitation letter to new user. When user click the link, it should redirect him to register or sign up page. But now it always redirect to ADFS sign in page. So how ADFS works for new user without username and password?

Thanks.