Open the discussion web live to set permissions on it. Select Tools | Security | Permissions and on the Settings tab choose Use unique permissions for this web, then Apply. On the Users tab select Only registered users have browse access, then click Add. Repeat for each user you want to add special permissions for, or just leave the fields blank and your users can register themselves automatically with a user's registration form. Don't worry, you won't have to add them manually. You can also specify who can access your protected web and what authorization privileges you choose to give them. Your host or Network Administrator must allow you access to open your web live on the server.

Save the changes to your web (making sure the registration form is uploaded if you use one).

If your platform does not support this feature or you need tighter security, you can also write custom scripts to allow access to your protected sub web (examples-VB Script or .asp, .cgi/Perl or .php), or use a less secure JavaScript on your pages if tight security is not an issue.