Unaware of such software, probably because I, like many amateur/pro photographers, am a solitary beast and am not used to sharing...
Here are some options:
* Some of us have tried a collaborative effort in Photoblog
(click here to see), which required signing out and sharing the same ID and password - cumbersome, but free.
* Photoshop Lightroom: Kelby offers a hint - each contributor can have his/her own library. LR is expensive, but fantastic. It cannot display the contents of more than one library at a time.
* Perhaps you can create a dedicated site on flickr, but the really important thing is to make sure everyone uses the same, precise, simple and comprehensive tagging rules - without exception. The alternative, of course, is to designate an administrator, but that isn't easy...
Hey, this could be a great opening for a very limited market, but developpers, off you go!