I would not suggest storing any user level files in the root level of the hard drive as this may have unexpected results long-term as OS X permission are designed so that User files are stored in ~/ and system files are stored in /.
Move the folder in question to the Public folder of your user account and modify the permission set using the Sharing Preference pane (Apple > System Preferences > Sharing).
Alternatively you can move the folder to /Users/Shared/.
If the file in question is an iPhoto library, follow this article: http://support.apple.com/kb/HT1198.
Hope that helps.
That still doesnt work for me.
I placed the folder files in shared. Gave the correct permisions to userA and UserB
The behavior above is still happining. UserB creates a file and userA doesnt have full permisions.
When placing the files into the shared folder I see that the everyone account that has permision fo reads and writes.
Is that everyone refer to local Macbook users and not over the network? (that would be defined in syspref file sharing)
Also does that apply to the guest account if enabled?
The everyone in this context refers to all users on this computer.
Once you have the folder in /Users/Shared drag the folder into the File Sharing Pane of Sharing preferences. You can then add / remove users you wish to access the folder of information and change their access privileges.
Good read: http://support.apple.com/kb/HT1549
Take a look at this recent thread:
As an alternative, you can store the shared files on a Mac-formatted external drive partition (a USB thumb drive will do), and in the external volume's Get Info window, check the box for "Ignore ownership on this volume".