Mac OS X 10.5 gives you the new ability to assign permissions to individual users—for example, if you want Jane to have access to edit a file, but not John. One way to set this up is from the file’s Info window.
To access this, select the file in the Finder, choose File -> Get Info and take a look at the Sharing & Permissions section at the bottom of the window. To set permissions for individuals, start by clicking the plus (+) button at the bottom of the window. This should bring up a list of users and groups.
Unfortunately, if you upgraded to Leopard from Tiger, clicking this button may instead cause the Finder to crash.
To fix the problem, you need to launch Terminal (/Applications/Utilities) and type the following commands (entering an administrator password when prompted):
sudo dscl . create /Groups/USERNAME GroupMembership USERNAME
sudo dscl . change /Groups/USERNAME RecordName USERNAME _USERNAME
Substitute the short name of your account for all occurrences of USERNAME. Repeat these commands for every account on your computer.