My wife occasionally uses my Mac and she has her own account/login so when she was logged in under her own account she couldn't access the iPhoto Library setup for use with my account. The iPhoto Library on my account is all our family photos and she wanted quick access to them. I set off to see if it was possible for multiple users on the same Mac to be able to share the same iPhoto Library from both of our accounts. After a little digging I found that it is actually pretty easy to do. This works on OS X 10.4+, if you're on an older system search elsewhere for a solution.

1) Enable ACL:
From a terminal session enter: sudo fsaclctl -p / -e

2) Create a new folder which will contain the shared iPhoto Library, I suggest: /Users/Shared/iPhoto Library
(You must create a new folder and not move or copy an existing folder.)

3) Add an ACL rule for each user who you want to be able to access the iPhoto Library:
From a terminal session enter the command below replacing USERNAME with the User's short name which you can find in the Accounts section of System Preferences.

(The command below is all one line, no spaces after the commas at the end of the first two lines.)

sudo chmod +a "USERNAME allow delete,chown,list,
search,add_file,add_subdirectory,delete_child,file_inherit,
directory_inherit" /Users/Shared/iPhoto\ Library

4) Now the iPhoto Library folder you created is ready to go, you can start using it by creating a new library in that location, or you can bring over an existing iPhoto Library.

NOTE: If you want to use an existing iPhoto Library, you must COPY it into the new shared iPhoto Library which you created, if you MOVE the files, it will not work. Copy and Paste, or Option+Drag/Drop to copy.

NOTE: If you hold down the OPTION key on your keyboard while launching iPhoto it will prompt you to select the iPhoto Library to use. After I copied my old iPhoto Library contents to my new shared iPhoto Library I was able to access the same iPhoto Library from both my account, and my wife's account.

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