Apply To Enclosed Items

You can use Access Control Lists to accomplish this. First go to System Preferences => Accounts. Unlock the padlock and click the + button. From the drop-down menu, select Group. This will create a new group of users. Give it a name like "sharegroup". Add the users you want to the group.

Then log in as an administrator and open Terminal. Paste in these two lines at the same time:

sudo chmod -R +a "sharegoup allow delete,chown,list,search,add_file,\
addsubdirectory,delete_child,file_inherit,directoryinherit" \

Then press return. Then drag the folder into the Terminal window and press return again. Enter your admin password when prompted.

That should do it. Any file you newly create or copy to anywhere inside that folder hierarchy will then have full read and write privileges to everyone in the group.

I'm pretty sure that was supposed to be "sharegroup", but you'll want to replace that with whatever name you gave your new group.

Before you get too far along with using an ACL, you should know that OS X 10.6.5 and 10.6.6 have a serious bug which results in the Finder duplicating a file's inherited ACL each time the file is copied, which could lead to the file failing to copy until the duplicate ACLs are removed. If you're not copying files back and forth very much, this might not affect you very often, but if you plan to copy files back and forth repeatedly, or if you're copying folders with several levels of sub-folders, this could become a problem for you very quickly.

You can take a look at this thread for more info:

http://discussions.apple.com/thread.jspa?messageID=12773904

If you try to copy a file and it fails with an "error code -41", then you'll know it's probably time to go in and delete some duplicate ACLs.

Yes, that was a typo. Whatever you use should match whatever is the name of the group you made in the first step.