Configure each computer to retrieve photos from the other. Each computer waits for the other to be on the same local network and to have new photos. It then copies the new photos from the other computer into its iPhoto Library's "Auto Import" folder. The next time you launch iPhoto (or switch to it, if it's already running), iPhoto automatically imports the photos.
Perhaps you have a Mac mini that you want to be your "home photo server". In that case, configure the Mac Mini to retrieve photos from all your other computers. The Mac Mini waits for any of the other computers to be on the same local network and to have new photos. It then copies the new photos from the other computer into its iPhoto Library's "Auto Import" folder. The next time you launch iPhoto (or switch to it, if it's already running), iPhoto automatically imports the photos.
 
iPhoto has a built-in sharing feature -- you can view photos in another computer's iPhoto Library, and even copy them to your own.
 
 
My wife and I each have a MacBook Pro, but we share our 2 cameras for taking family photos. We both want the photos, but sometimes we forget to import from the cameras into both computers. I always got the feeling I was missing some photos that she had, and vice versa. When I first used iPhotoSync, I found out she had 4,100 photos that I didn't have, and I had 7,000 photos that she didn't have!
 
Now we both have all the photos, and whenever one of us adds photos to iPhoto from a camera, the other person gets the photos automatically. If one of us adds photos to iPhoto while traveling, as soon as both computers are on the same local network again the other person gets the photos automatically.