can I share a photo library with two or more users on the same Mac?

http://support.apple.com/kb/HT1198

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For iPhoto 09 (version 8.0.2) and later:

What you mean by 'share'.

If you want the other user to be able to see the pics, but not add to, change or alter your library, then enable Sharing in your iPhoto (Preferences -> Sharing), leave iPhoto running and use Fast User Switching to open the other account. In that account, enable 'Look For Shared Libraries'. Your Library will appear in the other source pane.

Any user can drag a pic from the Shared Library to their own in the iPhoto Window.

Remember iPhoto must be running in both accounts for this to work.

If you want the other user to have the same access to the library as you: to be able to add, edit, organise, keyword etc.

Quit iPhoto in both accounts. Move the Library to the Users / Shared Folder (but note: some users find ongoing permissions issues with the Library in this location. It might be better to use an external HD set to ignore permissions, a Disk Image or even partition your Hard Disk.)

In each account in turn: Double click on the Library to open it. (You may be asked to repair the Library Permissions.) From that point on, this will be the default library location. Both accounts will have full access to the library, in fact, both accounts will 'own' it.

However, there is a catch with this system and it is a significant one. iPhoto is not a multi-user app., it does not have the code to negotiate two users simultaneously writing to the database, and trying will cause db corruption. So only one user at a time, and back up, back up back up.