How do I set up the administrator contact subfolders as an address book on a different mailbox in order to allow other users to add information?
Assuming you have an Exchange server you simply need to give the other users appropriate permissions (I like Reviewing Editor role for this) to the Administrator's root mailbox, and then to each of the Contact subfolders you want them to access.
Don't worry about the root mailbox; giving permissions there does not automatically filter down to other folders already in the mailbox, though it does filter down to folders created in the root after you've granted the permissions.
Once you've given the proper permissions to the Administrators folders, go to each user's system and find Tools > Services > Microsoft Exchange Server Service. Go into Properties and on the Advanced tab add the administrator as an additional mailbox to open.
Click OK back out to the folder view and restart Outlook. When it comes back up, open the folder list (View > Folder List) and you should see the Administrator's mailbox listed along with the user's. Expand the administrator's mailbox in the folder list and you should see only the contact folders you granted permissions for.
You can now drag and drop those to the Outlook bar, if you wish.