[Samba] using "@group_name" in smb.conf

[Darryl Harvey]

Hi all,

I need to give a group of users access to a file share that I am sharing
via samba.

In smb.conf I am using something similar to;

[accts]
   comment = Accounts Dept Share
   path = /usr/local/share/accounts
   valid users = @accounts
   public = no
   writable = yes
   printable = no
   create mask = 0765


The valid users line says who can access this share:  "@accounts".
I know the @accounts  = the "accounts" group.  

How/where do I add the actual valid usernames to this group? Within
smb.conf, or as a file (Where does the file sit and what is it called
(accounts ?)

I can't find the answer in the docs, I did try :-(

Help!

TIA
Darryl