Yes, you can set up a separate, private blog that requires members to log in. You can do this with a plug-in called Members Only - it's freely available from the WordPress plug-in repository.
Anyone who visits the site who's not logged in will be directed automatically to the login screen. After users log in, they can browse the site normally.
Based on your comments, it sounds like you don't need a private blog - you need a private bulletin board or forum.
I would recommend setting up a system like bbPress in a sub-folder on the site (i.e. http://tmpfire.com/forum/) and installing bbPress in that folder. You can incorporate bbPress with WordPress, or keep it entirely separate. But you can also keep it private.
This way, you can have a WordPress-powered website with a private information/discussion board for members only.