samba - Nov 2010 - Using groups in workgroup mode

I am trying to use membership of a UNIX group to define who has write access
to a particular share on my samba server but this is not working. The server
is in workgroup mode aka security = user mode and I'm using the line
'write
list = @staff' to define that only users in the UNIX group staff have write
access to a particular share. I haven't been able to tell from the
documentation or googling, but I think that this is failing because the
'write list' option directive is designed to accept SMB users and SMB
groups, not UNIX groups as options. Is that the case?

Thanks

Andrew.