[Samba] Newbie question - force file permission to user's secondary groups.

[Conta Falsa 337]

samba version is  3.0.28a-1ubuntu4.7
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I created users on both samba and the linux system, and created 3 groups on
the system. Each of these groups own a specific directory, the directory on
the filesystem belongs to root.groupfoo. On my smb.conf I gave each of these
groups write access to its directory (@groupfoo to the share /groupfoo). So
now every linux user belonging to groupfoo can write there. The problem is,
groupfoo is not the user's primary group, so the file is created with
permission user1.user1, and not user1.groupfoo, therefore, other users
belonging to groupfoo cannot edit or delete that file. I read smb.conf
manual, but found no option to enforce that if the top directory belongs to
root.groupfoo all files created under there will belong to
"userxyz.groupfoo", so I set on the filesystem each of those 3 directories
to be setgid, so now every file created under, say, /groupbar (belongs to
root.groupbar), has this permission: userabc.groupbar. I would like that the
file/directory created belongs to the user executing the operation, and to
the toplevel group owning that share, since a user can belong to 2 or all of
those 3 groups mentioned, knowing that every user does not have any of those
3 groups as primary group.

 Is this the right approach  or did I misunderstood the manual and I should
do this only on smb.conf and not have to enforce it on the filesystem?

 thanks for your time.