In iPhoto, I put a shared iPhoto library on a network external hard drive accessed by multiple computers in the family.
Even though two computer cannot access the library at the same time, the whole family can add to and edit the same (huge) library.
In Photos, I can't seem to select the "Photo Library.photoslibrary" from a second computer (via network shared drive).
If it's no longer possible to share a library from multiple computers, that would be a huge functionality loss for us – to not be able to keep one library for the whole family.
Our library is too large/expensive for iCloud Photo Library, which also would require all computers to sign in with the same iCloud account (no good for everything else).
UPDATE: since my shared Photo Library resides on an external hard drive, I tried attaching it directly to the second computer and then was allowed to select it (Option-launch Photos App) as well as setting it as the system library.
So it appears that the difference with iPhoto is that I can no longer share the library via network File Sharing (tried both smb and afp with no luck).
I really hope Apple allows this again as it's a hassle to detach/attach hard drives when we have high speed home network.
UPDATE 2: following Kevin's suggestion below, I tried creating a new Photos Library with only one photo added, and was able to access this library from another computer via network share. When trying to open the much larger library (converted from iPhoto library) from network share, Photos app showed a spinning wheel for a while and then returned an error message saying it could not open the library. The spinning wheel can persist for a long time, so I suspected it might be the sheer size of the library choking up the app. Indeed when I brought the computer into the same room as the wireless router, it did successfully open the library.
So bottomline is it still "works" provided the network speed is able to handle the size of the library.
Best Answer
Hold the alt (option) key while starting Photos App. You will be prompted to choose a Library.