How can I share Photos across multiple users on Apple Photos, without paying for iCloud?
I guess this is what you're looking for:


Enable iCloud's Family Sharing and iCloud Photo Sharing

If you have iCloud's Family Sharing and  iCloud Photo Sharing enabled, you'll see a special album called "Family" in the Shared tab of the Photos for OS X app, as well as the Shared section of the Photos app on iPhone and iPad. This album will automatically sync with all members within your Family Sharing group.
Share your photos

To add photos to the shared stream, select them and click the Share 
button. From there, choose iCloud Photo Library, and select the
Family album. The images will then sync over to your family member's
device.


If you need a deeper explanation: http://www.imore.com/how-use-iclouds-family-sharing-photos-os-x