Are you talking about WebDirect?
Assuming yes, you enable the guest account and add the webdirect extended privilege to the guest privilege set. Set the OnFirstWindowOpen trigger script to go to your login form.
Alternatively you can create an account with it's own restricted privilege set that includes WebDirect extended privileges, and under File > File Options, specify that account and password as the login. Again set the opening trigger script to go to a login page.
On your login page, have a button that runs a script with the "Relogin" script step, this will allow existing users to login. In your registration form, save the record of data and create the user using the "Add Account" script step to create their filemaker account.
I would have a look at the Accounts Module if you want to do user management inside of filemaker this way.