samba - Oct 2003 - Domain = security and valid users

Hi, I'm running 2.2.8a and am having some problems with user authentication.
I'm using security = domain.  This seems to work fine; until I try and
restrict access to the share.  As soon as I all a valid users = xxxxx to the
conf file; all authentication fails.  Does 'valid users' apply to the
domain
security model?  If not, how do I restrict access by user?

Thanks,
Vince