Understanding Mac OS X Lion Folder Permissions
As you might expect, folder permissions in Mac OS X Lion control who can use a given folder or any disk (or partition) other than the startup disk. You can set permissions for the folder’s owner, a subset of all the people who have accounts on the Mac (a group), or everyone who has the Mac’s address, whether they have an account or not (guests).
To help you get a better handle on these relationships, take a closer look at Mac OS X permissions, owners, and groups.
When you consider who can use which folders, 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 is the default owner of Shared folders and drives on that machine. The owner must be logged in to change permissions on his folders.
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.
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 purposes 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.