How share Photos library on the same Mac without iCloud

To share a library it needs to be in a separate disk partition or on an external drive with the "Ignore Ownership" flag enabled. A Shared folder does not suffice.

The procedure is described here for iPhoto. You can try the same for Photos. See this link: iPhoto: Sharing libraries among multiple users

Using the Shred Folder on your Mac was disabled for iPhoto and Photos libraries a couple of versions ago. The only way too do it now is to put the library on an external HD that is formatted exactly as shown in this screenshot and connected directly to your iMac via USB/Firewire or Thunderbolt cable:

There is one more big problem with sharing a Photos library on the same Mac - it is more restrictive than in iPhoto or Aperture.

If you want to share a library it cannot be enabled as your system photo library, so you will need a second library to use as your system library, or you could not use the Media Browser or Photo Stream. This is explained here: https://www.fatcatsoftware.com/powerphotos/Help/sharing%20a%20library%20between% 20multiple%20user%20accounts%20on%20one%20mac.html

Sharing a library between multiple user accounts on one Mac

Photos is not designed with multi-user use in mind, which makes it very difficult to create a setup where a single Photos library can be accessed from multiple accounts on your Mac. If you wish to share a library this way, the following restrictions will apply (for the below discussion, assume we have two users named A and B that we want to share a library):

1. Only one user account can ever have a given library open in Photos at a time. Before you can use the shared library as user A, you must first make sure to quit Photos as user B.
2. None of the user accounts that are accessing the library can designate it as their "system library". The system library is effectively open at all times by OS X in the background as long as that user is logged in, so even quitting Photos will not free up the library so that another user can open it. You must either designate a different library as the system library, or fully log out user B before trying to access the library as user A. Note that this also precludes having this library sync with iCloud.
3. The library cannot be stored on your internal drive, or any other drive where permissions are respected. Fighting against permissions on OS X is a losing battle; while you can play whack-a-mole trying to fix permissions on the library before you open it, something will eventually go awry and it's likely you'll be unable to access some portion of your library. You should store the library on an external drive, and in the Get Info window in the Finder, make sure that the "Ignore ownership on this volume" checkbox is checked.