|Nevermind. I found it!|
Should anyone surf into this thread with a similar problem, the solution is:
Log on as administrator, open up Windows Explorer, then choose Tools/Folder Options/View. Under Advanced Options, uncheck "Use Simple File Sharing (Recommended)".
Now (still as administrator) right-click the folder to which user accounts need access, and choose "Properties". A new tab is now visible: Security. Hooray! Click on it, choose User Accounts, and set permissions.
Always wondered what "Simple File Sharing" was. Now I know. Sigh.