No need to change the design of the Address Book to restrict users from sending to everyone. There's an easier and more supported way of doing this.
We had a group inside the company that frequently sent messages to customers. Rather than everyone creating an entry in their personal address book, we created a database on the server, based upon pernames.ntf template. Users who wanted to have access, were granted access through the ACL.
Then they could open it, view things, some were given Editor access to change the entries.
If users wanted to mail to that database, we created a replica on their hard drive (set up a schedule for replication). Add the address book under File->Preferences->User Preferences->Mail & News->Address Books.
Then type-ahead works and only 1 person needs to maintain the data. Works fantastic.
