Just to elaborate. /Users/Shared by default is accessible to every user. If you put your iPhoto Library in /Users/Shared and make sure you "fix" the permissions on the enclosed items, both users should be able to access it.
You'll also need to hold down option while you launch iPhoto on the second user and pick the option that lets you choose an iPhoto library. Navigate to the /Users/Shared/iPhoto Library, and you should be good to go.