On Windows, if you want to share a folder, you just right-click on it and choose Share.
No such luck on Mac. Very frustrating - it would appear that the only folder you can share is your home folder. Every now and then the usability folks at Apple just Get It Wrong, and this is one case (another case is their despicable Finder app, which I replaced with PathFinder and I am so much happier).
Some Googling on Mac sharing got me to a Mac Forum thread which suggested all sorts of twisty-turny configuration on the Mac, and then one quick email which said "why not just use SharePoints?".
Ah, all is goodness now...