MC is designed to maintain separate data for each Windows user. I have seen an "install for all users" option requested a few times, but I don't know if the developers have plans to add it.
As jroyale said, it is easy to set each user account to use a library that is in a common location. You can clone a current library and create a new library to a simple path like C:\MCstuff\Library with Library Manager. After that you can add that location to each user account. (File > Library > Library Manager > Add...)
However, there will be a problem with the thumbnail cache because the thumbnails will still be created separately for each user. That may cause MC to reset the thumbnails whenever it finds that they are out of sync. I.e. if one user has made changes to the library, another user still loads the old thumbnail cache on startup.
Fortunately, a Windows registry tweak can fix the problem. The location of the thumbnail cache is defined by a registry value. You can change the location to a common location. For example, to C:\MCstuff\TNcache. You must change the location for each user separately. It should be a folder that is on a fast local drive. The key is:
HKEY_CURRENT_USER\Software\JRiver\Media Center 14\Properties: Thumbnails - Base Path
You can copy the current library files and existing thumbnails to the new location if you want (naturally MC must be closed while you do that), but it might be a good idea to let MC create a new "clean" thumbnail cache.
This tweak changes only the thumbnail cache location. The user options will still be saved separately for each user.