Hide specific contact lists from users

It would be extremely valuable to have the option to set contact lists as private and/orhave the option to restrict access to users. 

For example, I may want two of my users to have access to all of our internal contacts, but want only one of them to be able to access our external/client contact lists. 

The implementation of this solution would significantly enhance organizational management and security, thus elevating the value proposition of this platform.