You should ensure that you have a column in your users table that will indicate if the user has the special access. This is a column that you should manually control yourself.
You should then start out by using the Security Assist wizard to create a user registration experience. This will create the pages for registering and logging in. You should default the special access column to the regular value, then update it manually for the user's that have special access.
Once you have this worked out you should ensure that you are storing the value from this new column on login. You do this by editing the authenticate user server behavior and adding a new session variable, then basing it off of the new special access column you added. When a user with this special access logs in they will have the session variable that holds this value.
The last part is to define your access rule based on the session variable. You will check it for the correct value. If a user meets the rules requirements then they will have access to any page that has the access rule on it. If the user logs in and does not have the correct value stored in the special access session variable they will not pass the rule and have access to the page. Please post back with any specific questions that you have about any part of this and I will help explain or help you figure out what is next.
