Workgroup: as far as I know you can't insist on a password to "enter the workgroup" - certainly I've never seen one, and given that they aren't generally hub / server based but distributed processors it'd be hard to see how you could set and enforce one.
PC: easy. When the drive is shared, there is normally an option to restrict who can access it. A quick google gives ways to protect for various systems:
Google: "password access to shared drive"[
^]
Submit an article or tip