1) Make sure your users are a member of a group. "Wheel" is the easiest...as you don't have to change much on the permissions side.
2) With a ZFS volume (assumed), you edit the permissions within the GUI by clicking on the key icon. The permissions you want to enable are all three checkboxes under "group" - if you added your users to a different group, then change it to match that. "Others" is what allows, essentially "everyone" to see - I usually leave these unchecked.
3) I've found it best/easiest to set up ZFS Datasets representing each share - this way I can manage permissions with a bit more granularity. This way, if you want only "User A" to have access to "Share 1", you set them as the owner of that volume, and leave only those checkboxes turned on....and if you want "User A" and "User B" to "Share 2", you do it with a group vs owner.