Permissions and samba when users access files

i am unable to share files onece it has been edited by a user so i implemented the following and it dose not seem to work .

sudo Mkdir /srv/myshare
sudo Chgrp group name /srv/myshare
sudo chmod g+w /srv/myshare
sudo Chmod g+s. /srv/myshare
Sudo chmod o-r /srv/myhsare

Result is

RWX. RWS. - — -
User. Group. Others

After doing this I still have an issue where permissions are reverted to the user who first open the file? How can I implement this to all sub folders

Is there anything I am missing?

Thank again for all your support