MacOS – Terminal command to change Privilege of “everyone” to “No Access”

Any folder added to your shared folder list is broadcast publicly. If you don't want it to be visible to guest users, remove it from your shared folder list. You will still be able to access it by logging in as a registered user, using your user name and password. If you log in as a registered user, you will be able to see all your files, but a guest can only see files in the shared folder list.

While still not understanding the reason this occurred, I was able to make the problem go away by explicitly adding the user to the inherited group in Workgroup Manager, verifying it with dsmemberutil, reconnecting to the AFP share, disconnecting, and then removing the explicit group assignment. After all that, dsmemberutil still shows that the user is in the group.