Without knowing how you and others are actually accessing the NAS (browsing to network share or shortcut, mapping a network drive, etc.,) and what format (Fat32 or NTFS) the NAS drive(s) are it is difficult to suggest how you might proceed.
If NTFS you will likely have to edit the NTFS Permissions (Security Tab) in the properties of the folder, individually per user, in the properties of the parent folder by clicking the Advanced button there. And in order to do this successfully you will likely need to break inheritance of that folder from its' parent container by clicking the Change Permissions button and unchecking the box that says Include inheritable permissions from this object's parent (be sure to click the Add button when...