I think you are searching for /usr/local/share/ but it is hard to answer since this depends on what kind of file your are planning to "share" between users.
But if we are talking about office files or something like that maybe you should use use some kind of revision system like subversion or git. And then the users will have a checkout/clone in their homedir.
Update: A way to make this a little bit better could would be that every user gets it's own subdir in the shared folder. He is allowed to write in his own folder but not the other subfolders. And all users are allowed to read from all the directories. That way you don't have to think about file collisions if two users use the same filename, or deletes the colleges files by mistake.
Btw the idea behind /usr/ is described in the Filesystem Hierarchy Standard ( http://www.pathname.com/fhs/2.2/fhs-4.1.html )
-"/usr is shareable, read-only data."
So I would probably use a dir in either /home/ or /var/ instead...