Answer by Paweł Brodacki for Setup Directory Permissions For Multiple Users To Read/Write

You have better part of the solution figured out, the only thing left to do is correct permissions for things created by root. There are at least two approaches

1. You can modify the root-side process to call chmod after modifying a file. I do not think it is applicable to your case, because then you'd probably just call chown and chmod and didn't set up sgid directory.

2. You can modify umask for the root process. I think it would be better to modify umask just for this process, not for root environment, because sometimes root touches files other users shouldn't mess with, and it's usually better to remember to open access than to close gaps. Therefore, you could add umask 0007 in front of call to the binary that runs as root.