samba - Sep 2003 - NT domain user and local unix group

Hi,
i need to add NT domain user to a local unix user; the samba server 3.0
isn't
a PDC or a BDC but only a domain machine member with on share directory. Now 
i use ACL on filesystem to setting permissions.
What i need is to setting this permissions based on local unix group and add 
NT domain members on this local group.
Winbind works fine, so I can see domain users and domain groups, even with 
wbinfo and getent commands (I setup the nsswitch.conf file).
I try to add a domain user to local unix group with gpasswd but winbind ignore 
it so i can't access to a directory with ACL for this local unix group.

Thanks in advance,

                            Simone