As is the case with other objects or files that are shared between users, (Hint: Xrefs, Data Shortcuts, Images, etc.) the drawing does not store these files/objects inside but rather remembers where are they coming from by using paths. It is important to know that Parts Catalogs are external to the drawing, too. By default all pipes are stored in this location: C:\Documents and Settings\All Users\Application Data\Autodesk\C3D 2010\enu\Pipes Catalog.
This Catalog Manager (a person assigned to manage parts) can use different location and you, as user, can manually point to that location. That way you will always have updated version of catalog files. This task is easy to do. All you need to do is to select Set Pipe Network Catalog from Home ribbon.
Another thing that's important to note is that in multi-user environment it's easy to create unwanted changes and that also affects other drawings that referenced part catalogs. In that case, too, Catalog Manager comes handy. When the drawing is opened on a different computer, by default program looks for parts at C:\Documents and Settings\All Users\Application Data\Autodesk\C3D 2010\enu\Pipes Catalog. That can cause that some parts are not found or that some parts look different in the drawing.
So when there are some catalog changes this Catalog Manager can distribute them or just manage it in the central location. As you can see there are many reasons to assign one person to act as Catalog Manager.
Off course, there are situations where user creates or receives new part and would like to share it. Then that part can be sent to Catalog Manager to be added to catalog.
If you, however, decide not to utilize Catalog Manager you must know that there must be lot of good communications between engineers-which is sometimes hard to do since some of them are in and out of the office all the time.
What if users do not want to change this default location? Some people prefer not to change this location, then Catalog Manager can distribute these parts to all users to be copied in the default location on their C: drive. When sharing a drawing with other users set part catalog to the same location to prevent unwanted part changes. If you happen to have part that's used in the drawing from your own catalog and then you send that drawing to other users they will be able to open that drawing but the parts that don't exist in the other users catalog will be replaced by the closest part Civil 3D can find. I have seen too many of these issues.
So solution to all of these issues can be a Catalog Manager!