File Sharing Permissions in Mountain Lion
When you consider who can use which folders via files sharing on your Mac running OS X Mountain Lion, three distinct kinds of users exist on the network. Here’s a quick introduction to the different user types:
Owner: The owner of a folder or disk can change the permissions to that folder or disk at any time. The name you enter when you log in to your Mac — or the name of your Home folder — is the default owner of Shared folders and drives on that machine.
Ownership can be given away. Even if you own the Mac, you can’t change permissions for a folder on it that belongs to another user (unless you get Unix-y and do so as root). The owner must be logged in to change permissions on his folders.
OS X is the owner of many folders outside the Users folder. If OS X owns it, you can see that system is its owner if you select the folder and choose File→Get Info (or press Command+I).
Folders that aren’t in the User directories generally belong to system; it’s almost always a bad idea to change the permissions on any folder owned by system.
If you must change permissions on a file or folder, select its icon and choose File→Get Info (shortcut: Command+I), and then change the settings in the Sharing & Permissions section at the bottom of the resulting Get Info window. Do not change permission settings if you’re not absolutely sure of what you’re doing and why.
Group: In Unix systems, all users belong to one or more groups. The group that includes everyone who has an account with administrator permissions on your Mac is called Admin. Everyone in the Admin group has access to Shared and Public folders over the network, as well as to any folder that the Admin group has been granted access to by the folder’s owner.
For the purpose of assigning permissions, you can create your own groups the same way you create a user account: Open the Users & Groups System Preferences pane, click the little plus sign, choose Group from the New Account pop-up menu, type the name of the group, and then click the Create Group button.
The group appears in the list of users on the left, and eligible accounts appear with check boxes on the right.
Everyone: This category is an easy way to set permissions for everyone with an account on your Mac at the same time. Unlike the Admin group, which includes only users with administrative permissions, this one includes, well, everyone (everyone with an account on this Mac, that is).
If you want people without an account on this Mac to have access to a file or folder, that file or folder needs to go in your Public folder, where the people you want to see it can log in as guests.