アクセス権に s ビットを立てる

% mkdir aaa
drwxrwxr-x 2 masutaka 512 Apr 3 10:42 aaa/
% chmod g+s aaa
drwxrwsr-x 2 masutaka 512 Apr 3 10:42 aaa/
% chmod g-s aaa
drwxrwxr-x 2 masutaka 512 Apr 3 10:42 aaa/