Important: If two or more users attempt to use the same iPhoto library and the library is not on a volume where permissions are ignored, unexpected results may occur. This includes the inability to see or edit certain photos, or the inability to open the iPhoto library. Follow the steps below to ensure each user has read/write access to the iPhoto Library.

As a precaution, you should make a backup of your iPhoto library before following these steps.

To share your iPhoto library, you need to move the iPhoto library to a volume where permissions are ignored.

Move the iPhoto library to an external drive

1. Connect an external USB, FireWire, or Thunderbolt drive which has enough free space to contain the library.
2. Select this drive in the Finder.
3. Choose File > Get Info.
4. In the Info window, find the Sharing & Permissions section. You may need to click the disclosure triangle to see the details for this section. 
5. Click the lock in the lower-right corner of the Info window. Enter an administrator name and password when prompted.
6. Select the option to Ignore Permissions on this volume (usually selected by default).
7. Copy your iPhoto library to this drive.

Important: If you've selected this volume for use as a Time Machine backup, you will not be able to select the option to ignore permissions on the same volume. Select a different volume for the destination of your shared iPhoto library.

Keep in mind that while one user is viewing this library from iPhoto, another user will not be able to open this library at the same time. Instead, the following alert message appears indicating that the library is already in use. You'll need to quit iPhoto from the other user account before the next user will be able to view or edit this library.