Folder on Network Drive Invisible.

Please help,

I am runnign XP Pro SP3 clients with a Windows Server 2008.
I am in the process of permissioning folders on the D: with Global Security groups and adding users to them.
Annoyingly when I assign a group to the folder the folder disappears from view on the user’s network drive. If I permission the individual it’s fine. Can anyone help? Choclate bar top the winner.