Mac File Sharing allows you to share files and folders on a Mac with other Macs and PCs on your network.
You can directly select a folder in the Finder to share on Mac OS X 10.8 Mountain Lion.
How to quickly share folders on Mac Mountain Lion?
Select the folder from Finder, choose Get Info from the File menu, and enable “Shared Folder” to share it.
Alternatively you can simply Control+Click the folder you wish to share and choose Get Info to display the Get Info window. Then look under the General section and check the Shared Folder checkbox.
When you no longer wish to share a folder on Mac, deselect “Shared Folder” to stop sharing the folder, after that the file sharing will be turned off..
Note: you need to enable the file sharing first if not yet. See the instructions here Steps to Enable File Sharing on Mac OS X 10.8 Mountain Lion