sharing records with assigned users

Ave, I'm working on a Phonebook Application for my company. Much of it's functions and application are pretty much clear and workable for me. But there's one aspect which I'm trying to figure out. It's a multi-user application where there's 2 kinds of users... Admin & User. The User can only view, add, edit, browse through his own records. Each user does get a public & private option, wherein the user can make the record either private, so only he can see it, or public which anybody can view. The Admin can do absolutely anything. There is one feature that Admin gets which I'm trying to figure out. When the Admin is adding a record, he gets to decide which users he wants to share the record with. He can choose particular Users by clicking on their name to share that record with. How do I make that happen? Any suggestions? Thanks.