I've tried to understand this whole concept. Official Apple docs and manuals (as well as online tutorials -- they just copy&paste Apple docs) were completely useless. It took me an hour to finally figure it out on my own.
Here's what you do:
1. Go to System Preferences and click on Users & Groups;
2. Select the Guest User.
You might need to click on the lock icon at the bottom and enter admin password to make it selectable;
3. Check the "Allow guests to connect to shared folders". Do not touch the first checkbox that says "Allow guests to log in to this computer" because it has nothing to do with network access;
4. Head back to the System Preferences and click on Sharing.
5. Enable File Sharing by clicking on the checkbox left of it.
6. By default, there's gonna be only one folder enabled for sharing — your Public folder. Select that folder and change the access rights for Everyone to whichever you need (Read Only, Read & Write or Write Only). If you need to share additional folders, just drag-n-drop them to the Shared Folders list and change the rights the same way.
Repeat those steps on all your Macs if needed.
That's it. We've just enabled folder sharing over local network without creating any users.
If you need to further specify which folders get access over network