| Workgroup Manager is the tool you want to use to set the permissions on the data that you are sharing, unless this is Leopard, then you want to use Server Admin. You could also use the command line, but, if you are not too familiar with the use, WGM is the best way to go.
What OS version are you using? Do you have ACLs enabled? You can use the Effective Permissions Inspector to check the permissions your users would have.
What type of directory service are you using?
Michael |