[Samba] force all connections that come from a windows group to be a specific user and problems with net groupmap add

[g p]

Does anyone know of a simple (share) mechanism that doesn't require setting
up a complex ldap, winbind, pdc, etc samba configuration
that will allow users connecting to a samba share to always be forced to be
a specific unix UID?  The idea is to have
the linux data owned by a particular linux user, and be able to control
write access to that data by adding any arbitrary
windows user to a specified windows user group.  The reason why ldap,
winbind, etc is not a desirable config is that
this could be deployed at sites that have stand alone workgroups with a
single user to very complex ads or ldap
configurations with many thousands of users, and we would like to have a
simplistic one size fits all access control
method.  Note that this doesn't have to conform to ideal security protocols,
or what a zealot would consider ideal configurations
or IT practices - functional simplicity is preferred.

Also - on another related, but separate note, I cannot seem to get the net
groupmap add command to work correctly.
I have a stand alone samba server, and a windows box that I have local admin
permissions to - I add a new windows group
and new unix group on the unix side - run the command net group add
ntgroup="<windows_grp_name_here>" unixgroup="<unix_group_name_here>",
but when I try to actually write to the share with a windows user that is in
the supposedly mapped group, I only receive a permission denied error.
a net groupmap list shows that the mapping did take, but again, it doesn't
appear to actually function as I would have expected.

Any hep would be great on either or both of these questions.