changing group ownership of files

[dowlime]

We are testing samba and are not sure if it is possible to change the
group ownership on a file from the NT permissions dialog.  The
permissions box shows 3 ACL's, one for UNIX owner/group/other.  However,
we can't seem to change the group ACL to a different group.  Removing
the group ACL and Adding a new group ACL for a different group does not
work.  Is the group ownership only changeable from UNIX?  If it is
changeable from NT, how is this done?