Will these users register through your SSO? Or are you trying to have SSO and a sign up form? You cannot have SSO and a separate sign up form. You will have to modify your SSO and switch it to a custom oauth2 provider then you can also have a sign up form for new users who are not going to use the oauth2 provider.
Then you can just make your forum public and anyone who wants to post will have to create an account.