Lets say user1, user2, and user3 already belong to the same group, staff. We can set the group permission to all their files like this:
chgrp -R g+wrX staff ~user1 ~user2 ~user3
That grants read, write, and execute access recursively to all the users' files and folders, to anyone in the group staff. You obviously can be more selective as needed. Now here is the trick that grants access in the Finder: Create a directory somewhere, such as /Users/Shared, for each of your users:
% cd /Users/Shared
% mkdir user1 user2 user3
% chmod g+wxr staff user1 user2 user3
% ln -s /Users/user1/* user1/
% ln -s /Users/user1/* user2/
% ln -s /Users/user1/* user3/
This step creates a set of Unix symbolic aliases that "mirror" the contents of each user's actual home directory. Now to use the Finder to access user1's stuff, go to /Users/Shared/user1. It will have everything you want, but no red minus signs on the folders barring your access.
Of course, this does not dynamically update the contents of the ~user* folder when the user adds new files and folders to the top level. But you can re-execute the above at anytime to refresh it, or for added kludge, please make a folder-action script to do it for you.
[robg adds: I haven't tested this one ... there's an older hint with some similar, but not identical, discussion.]

