Use SGID permission mode on parant directory. After doing this, any newly created file or sub-directory will inherit the group of parant directory. To do this:
chmod g+xs /path/to/dir
Or simply use:
chmod g+s /path/to/dir
--- INFO. ADDED -----
Sticky bit is different from SGID. But it will additionally help to protect content of directory. After setting it, only root, owner of parant dir. or owner of file/subdir. can alter the file/subdir. To set it:
chmod a+t /path/to/dir
Note: /path/to/dir is full path of parant dir. on which you will set SGID or sticky bit.