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).
There's an option for and access control entry (the thing you created in Get Info) to be inherited, but Get Info doesn't give you the option to enable it. Also, you'll want to make sure you have full access to items she's added/edited there, and the easiest way to do this is to create a group for the two of you, and allow access to all group members.
Create a group (in System Preferences -> Accounts pane), and check the boxes to make both yourself and your wife members.
Open the Terminal utility, and run the command:
chmod +a "group:admin allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit" /path/to/the/folder
(replacing /path/to/the/folder with the unix-style path to the folder you want to grant access to. If you aren't sure what that is, just paste in the rest of the line, including the space before /path/to/the/folder, and then drag-and-drop the folder from the Finder into the Terminal window, and it'll paste in the path for you.)
The access control entry you just created will be applied to new files and subfolders created inside that folder, but won't apply to the files and subfolders that're already there. So, Get Info on the folder and use the Apply to enclosed items option to apply it to the existing items.
Best Answer
This is actually the correct mode of operation for your OS. This improves the security of the operating system by ensuring that all expanded permissions must be intentionally set, rather than simply inherited. However, you can change this by using Access Control Lists (ACLs) in OS X.
Using the Terminal app in Application > Utilities, navigate to the parent folder in question using the
cdcommand. Then, run the following commands:The first commands allow everyone in the folder's user and group to access all files and folders in the directory, and allows people outside the group to read the files. The second sets those permissions to inherit to all new files and folders within the shared directory.
For more information on
chmodsyntax and man page, look here: http://ss64.com/osx/chmod.htmlAnd here is a general
chmodtutorial: http://catcode.com/teachmod/