On my Ubuntu server, I've set all my shares up in /etc/samba/smb.conf, so I'm not sure why they're in a different location..
I used the VALID USERS = parameter, per share, in the smb.conf in order to limit who can access what share...
That way, only my user, and root can access my unorganized "Stuff" share.
[Stuff]
path = /media/Storage1TB/STUFF/
available = yes
valid users = billinja, root
read only = no
browseable = yes
public = no
writable = yes
and in this example, my wife has access to our photo collection.
[Photos]
path = /media/Storage1TB/Photos
available = yes
valid users = billinja, root, april
read only = yes
browseable = yes
public = no
writable = no
These users need to be added as samba users, with samba passwords on the system, however they don't have to be personal accounts. I could just as easily set up a STUFF user or PHOTOS user, and given it a password that I would share with others. That just isn't as secure.
I'm sure there is also ways to do this with groups, but I haven't had the need to look into that yet.
Hopefully this helps - if not, I'm sure someone will correct me