You CAN send emails to members of a group, but apparently not create rules for public folders, as alluded in other posts (this one for example
viewtopic.php?t=8678&highlight=sxaa). Also, you can't have folders for groups.
But most likely it is not that difficult to do exactly what you want just by using the proper arguments to sxaa (I don't use sxaa that much).
./sxaa --user <user> --redirect <group name> --retain
Run sxaa with no arguments or options to see the full help or check
http://downloads.scalix.com/ark/sxaa.readme
In your case, you can create an *user* with name <department_user@domain.com>, as well as create a *group* with all member of the department called <hidden_dept_group@domain.com>.
This way you can "share" the <department_user> Mailboxes (Inbox, Sent Mail, Calendar, etc) and make then available as a public folder. In fact, you could just as well edit the permissions or delegate privileges from <department_user> to other users.
The net effect is that any emails send to <department_user> will automatically be filed in that user's Inbox as well as sent to everyone in the <hidden_dept_group>. Well behaved users can look at the "shared folder", as you intended originally. No so well behaved get included in the group and get the reminder in their inboxes.