OS X is by default set up so each local account can have quick network access to the account's home directory as well as locally attached filesystems to which the user has access. While this is convenient for the home user, there may be times when you would like to quickly share files with other people on the network, without either granting them access to your full user directory or continually moving files to the "Public" folder in your user account.
While the "Public" folder is the default location to put files for others to access, you can also just as quickly set up a shared folder to have in a convenient location such as your Desktop, or set any folder to be temporarily shared for the file transfer.
To do this, you will first need to have File Sharing enabled on your system, so go to the "Sharing" system preferences and check the box next to "File Sharing." You can use this system preference pane to help manage all the shared folders on your system, so if you have multiple shared folders that you have lost track of, you can easily go here to change the settings for them.
With File Sharing enabled (it only needs to be enabled once), just follow these steps to quickly share any folder:
1. Get info on the folder
2. Click "Shared folder"
That's it! The folder is now shared and should appear in the "File Sharing" section of the "Sharing" system preferences. If the permissions are the default where your account has read/write access, and "everyone" has read-only access, then anyone on the network who connects to your system will be able to view the contents of the folder. They will not be able to write to your system unless you either change the permissions for the "everyone" group or give a specific person write access.
Once the folder is shared, instead of having everyone be able to read files you can customize permissions so that only specific users have access. To do this, once you have authenticated by clicking the little lock, first set the "everyone" group to "no access" so guest users on the network will not be able to see your files. Then click the "plus" button and you will be presented with a list of available people, which include local user accounts as well as contacts in your address book.
Keep in mind that the people in your contacts list do not have local accounts on the system, but are available so you can easily give them a "sharing only" account from this interface. Sharing-only accounts will not be able to log in to your system locally and run applications. They can only access files for which they are permitted from other computers on the network.
Select a desired user, or click the "New Person" button to add a user name, and you will have to then give that user a password (make sure you tell that person what their password is). With the password set, the new user will appear in the users list, and clicking "Select" will add that user to the info window.
From here, you can then set special permissions for that user account, allowing them to either read, write, or both read and write to the folder.
As long as these permissions are set, the users and groups will have to follow them. There is no need to close the information window for any changes to take effect, which is convenient because you can keep the info window open to use as a control panel for managing access to that folder, especially if your sharing setup is to be temporary.
Remember, that if you forget which folders you have shared, you can always go back to the "File Sharing" section of the "Sharing" system preferences and manage your shared folders from there. NOTE: Newly created user accounts will need to be removed or otherwise managed via the "Accounts" system preferences.
Access from Windows
OS X has built-in support for Windows file sharing, which will not only allow you to access Windows computers but also share files with them. By default only the Apple Filing Protocol (Mac-only sharing) is enabled, so you will need to specifically enable Server Message Block (SMB, or Windows sharing) if you want to enable access to and from Windows machines.
To do this, go to the "File Sharing" section of the "Sharing" system preferences, and click the "Options..." button. Then click "Share files and folders using SMB (Windows)" option, and check the box next to the accounts that you wish to grant access to the computer via SMB. Doing this will require you to enter the account's password, so do this for the current and new accounts you wish to give access, and they should then be able to connect from Windows computers on the local network.
In this window you can also enable access to your shared files via FTP, so if for some reason Windows sharing does not work or is not available on networked computers, you can still transfer files between systems using the old FTP protocol.
Advanced tip: If you have "Remote Login" enabled in the Sharing system preferences, you can still use the "sftp" protocol for connecting (via the Terminal or other client that supports it). Keep in mind this will only work for local user accounts, and will not work for sharing-only accounts.