This is real basic, and won't cover all the nuances, but it will get you started.

First, you have to turn on advanced permissions. This is a one way street and once you do this you can't revert to basic permissions.

Then you can add pages that have info on them that you only want certain user to be able to access, and lock those pages down to only them and admins. You would set the permissions for those pages on the page while editing, or from the dashboard.

You can add users manually and give them the login credentials, or set up a single page for the login that is branded like the rest of the site. It all depends on how far you want to take it.

I set up a site for someone that allows access to certain pages upon registration here:

http://www.appsforinvestigators.com...

You can also add users, create a specific group for them, add users to the group, and then give that group specific page permissions.

Advanced permissions
http://www.concrete5.org/documentation/using-concrete5/dashboard/sy...

Single pages
http://www.concrete5.org/documentation/developers/pages/single-page...

By no means is this a tutorial on how to do this, and it really depends on how "glossy" you want to to come across. In other words, you can just have them login to the same admin login as the default one, or you can go the extra mile and build a single page login page that fits with your theme and site.

Hope this get you pointed in the right direction.