yes, you can do it. You only need to enable your "Hidden pages" extra:
1) Go to "Manage extras
" and click on "Add extra" tab.
2) Then select the "Hidden pages" extra to enable it.
3) Configure the extra and click on "Save".
Once your extra is activated go to the following link to create a new user:
Click on "Manage users" tab and complete the form with an username and password. You can create as many users as you want.
Now you can choose which pages you want to make private and the users who will can access them:
1) Go to "Manage pages" and select the page you want to make private.
2) Under the name of the page you'll find the "Show options" button, click on it.
3) Now you can set you page privacy:
You can make it public, private for all of your "Hidden pages" extra or private only for 1 of the users. You can't make it private for a gruop of users.
If you want your users to sign up you can use your contact form customized fields to ask your visitors for an username and password and then add them manually on your Hidden pages extra. It can't be automatic.
Hope that helps!