more on umask

Inspired by Randall Schulz contribution, I would like to ask how would you set the umask (if that is how it is done) so that newly created files/directories keep the group and privileges of the container directory and not necessarily the group of the creator? By way of example, my default group may be "admin" but I want to create files in a directory owned by group "sales". I want that file to be readable by the sales group but I do want to have to do to a chown or chgrp each time I create a file. Thanks Eddie