Share files/directories by setgid

>To allow write access to a specific group, shared files/folders can be made writeable by default for everyone in this group and the owning group can be automatically fixed to the group which owns the __parent directory__ by setting the setgid bit on this directory: > >chmod g+s our_shared_directory > >Otherwise the file creator's default group (usually the same as the user name) is used. The above quote is from Arch Linux Wiki . It is not clear to me how to make shared files and folders. Say user __A__ and __B__ both belong to a common group __G__. Now how do I create our_shared_directory such that by default everyone in __G__ has write permission? Second, why do I need setgid on our_shared_directory? Why do I need to make the owning group fixed to group of the __parent directory__ of our_shared_directory?