Sharing photos.app library across multiple machines
According to this Apple Support Document:


  Important: If two or more users attempt to use the same iPhoto library
  and the library is not on a volume where permissions are ignored,
  unexpected results may occur. This includes the inability to see or
  edit certain photos, or the inability to open the iPhoto library.
  Follow the steps below to ensure each user has read/write access to
  the iPhoto Library.


Essentially, a single Library must reside on a volume where permissions are ignored. A NAS or a shared internal/external non-System HD attached to your Mac Pro be a good start. You will not be able to share the Library from the internal boot drive as it cannot be set to ignore permissions.