You can't do this with traditional POSIX-style permissions, but you can with inheritable access control entries. To allow read+write access for the entire "staff" group to everything in /Users/Shared/reallyshared, you'd use:
sudo chmod -R +a "staff allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit" /Users/Shared/reallyshared
The file_inherit,directory_inherit part means that this access control entry will automatically be added to new files & folders created inside this folder, but it doesn't automatically apply to items already there (that's why I added the -R flag to chmod -- that applies it to everything currently within the folder), and it won't be applied to items created somewhere else and then moved into this folder (I don't know of a way around this, sorry).
Best Answer
Right click the directory or Command + I after clicking on it, there will pop up a window like this.
Clink on the padlock in lower right corner and type your password. Now you can change permissions for this directory. If you want everyone to be able to read and write, click on the privilege provided to Everyone and there will appear a dropdown menu from which you can choose what permission to provide.
I hope that helped.