I have the following share defined in smb.conf:
path = /home/user/data
writeable = yes
valid users = me, them
force group = them
create mask = 775
The "force group" option seems to work, but the "create mask" option does not. All files are created with the 755 mask instead of 775--thus not giving "them" write access to files that "me" creates while using a Windows workstation to access the share.
Note that I have restarted the samba server (service smb restart) to make sure the settings take effect, but it still doesn't work.