Help with FreeNAS 8 & CIFS Share

I have set up a PC with FreeNAS 8 to use as a network storage device accessible by OSX, Windows XP/7 and Linux machines. As a result, I've chosen to share my volumes as CIFS shares.

I'm starting with just one HDD for starters but have run into a problem: no matter what I do, my CIFS share is accessible with no authentication. I can access it just fine from all of my computers, but the FreeNAS box is not requiring me to login as one of my defined users. The FreeNAS box is behaving as if I am always a guest with full read/write privileges.

This is despite creating a group, and adding several users with passwords to the group. I also set the following settings as per several "how-tos" I've found online:

Volume permissions
owner (user) = nobody
group (user) = [the group containing all my users]

CIFS Settings
Authentication Model = local user
Guest Account = nobody

I also made sure that the "Allow Guest Access" and "Only Allow Guest Access" boxes are NOT checked.

How do I force the FreeNAS box to require that I login as one of my defined users in order to connect to the CIFS share?

All help is greatly appreciated!