How can a file moved to a directory with GID-bit set inherit the group ownership
Solution 1:
The setgid does affect the newly created files (using touch, cp), and it does not affect the already created files (using mv).
There are several solutions like the ones posted here. Also, you can add a cron job to periodically chgrp all files / folders under your shared folder.
Another possible solution is to use incrontab -e to fix group ownership when a file is moved to your shared folder. Be careful, this triggers events only for shared folder itself not any other subfolder.
/path/to/shared/dir IN_MOVED_TO /bin/chgrp users $@/$#