http://wiki.dreamhost.com/Unix_Groups (basically the same thing)
However, this isn't possible on shared hosting accounts due to recent security measures we've taken, part of which was locking down home directories in a way that wouldn't allow those unix groups/symlinks to work. A disclaimer: We can't really provide support for this, and it definitely may break things.
I highly recommend creating the new user and then going to the Fully Hosted settings of whichever domain, and select the new user there. A little box will come up asking if you want all the content moved. Keep it checked, save your settings and then just share that specific user/pass info with whomever else might need to work on the site/in that directory it has access to. The password can be updated again later or the site can be moved back to one of his other users at any time. The user won't have access to any of their other sites unless they manually go in and move those sites to that user too.