What dragon suggested (copying the Library) will give you duplicates, but it will also make any changes not propagate. (So, for example, if you add a picture in 'your' library, it will not be in the other user's library.)
The easiest solution is to move the Library folder into a shared folder (I have mine in /Users/Shared/Pictures/iPhoto Library) then make an alias to that folder and put the alias in each user's home/Pictures folder. I've attached a screenshot showing this. My real library folder is in the 'Shared' user folder, with an alias in my Pictures folder. (My wife's account has an alias to it, too.) This way, any changes either one of us makes to our photo library are good for everyone. (I do the same with our iTunes library.)
(My photo library is 7792 items, 13.4GB, all but about 20 taken with my digital camera. The remaining 20 are scans of older physical photos. My iTunes library is 3593 items, 17.38GB, all either ripped from my physical CDs that are in a box in the basement, or downloaded from iTMS. (1.96 GB in Podcasts, 2GB of Audiobooks, 328MB of purchased video, 1.64GB of purchased Music that doesn't include the audiobooks or videos.)