Possible in Snow Leopard, BUT...
Simply create the user, set to "sharing only", accept the changes, then command-click the user in Accounts preference pane, choose "advanced options", and change "login shell" to "/bin/bash".
Note that the privileges of this user seem fairly widespread - from the terminal at a nearby Ubuntu box I logged in and I tried a few things, and was even able to delete files that belonged to me (me as in GUI me, not to the "sharing only" user) - I was merely prompted "override rw-r-r clancy staff for [file]? y/n".
dseditgroup admin yields myself and root as members, not the new user. Not sure why he's so damn powerful, but perhaps this is why default shell is none!
Clancy