Hey,
Revo comes with a powerful User management addon called login. You’d probably first need to install it through package management within MODx. Then you’ll need to assign a permission group to any document you need to password protect. This assumes that you’ve already created resources groups. When you create those groups , you can specify whether a password is required or not to view documents that belong to a given group.
Then you will need to place a login snippet call wherever you’d like the login form to appear.
I hope this gives you a general idea behind getting this done.
You can find a better documentation on this addon here:
http://rtfm.modx.com/display/ADDON/Login