Use a Windows host Computer Manager MMC, connect to the NAS server.  You can then add local users and groups from there.
I'm at a complete loss trying to figure out how to setup a basic NAS on my new PS500. When defining a new NAS server NOT on an AD I have to define an Administrator password. Seems I'm then stuck with administrator/password for all shares in that NAS instance after?! I can't define users or change the password anywhere. This seems ridiculous to me! I've never seen a storage device not allow some type of local users per share etc. And yes, I have an AD & a UDS LDAP. That's irrelevant for this use case at this time. Equal Logic with a Fluid FS could do this, Tegile could do this, every QNAP & Synology etc. Honestly never heard of a storage device that can't, so I didn't consider it when implementing the PowerStore.
CIFS/SMB Local Users