Is the other user's iPhoto library on a different computer on the same network? Then the other user should enable "Sharing" in the Preferences and if you enable "Look for Shared Photos" you can download them.
Sharing a library on the same computer is more difficult:
See this article: iPhoto: Sharing libraries among multiple users
If you both are using the latest iPhoto version with MacOS X 10.8.2. or later you can exchange photos using a shared photo stream: http://support.apple.com/kb/TS4379