[Samba] windows permissions

[build]

G'day All,
After searching the archive I am no closer to achieving what I want. 
I've seen lots of posts that suggest this can be achieved but the 
methods used don't work for me. I won't pretend I understand linux or 
samba permissions because every time I think I've got it, I turn out to 
be wrong. So here goes.

Let's say I have 3 users & 2 groups.
each group has it's own directory.
user1 is a member of grpA & grpB (needs rwx access to grpA & grpB 
directory & all files).
user2 is a member of grpA (needs rwx access to only grpA directory & all 
files.
user3 is a member of grpB(needs rwx access to only grpB directory & all 
files.

When a user creates a file I want the file to have -rwxrwx--- (0770) and 
the group to ?own? that file so others in the group can edit it. No 
matter what I try any files created belong to the user with -rwxrw---- 
(0760).

Do I need to change umask?
How should the groups share be configured in smb.conf?
Are there any other changes in smb.conf required?

Thank you in anticipation,
Lindsay