User strategy: an example
Consider how your users can best make use of the directory. In some cases, simply searching the directory each time an email is sent is simple and effective (Email clients such as OS X Mail, since they auto-complete, make this easy, although it takes a few seconds to search).
In other cases, it's better to keep some or many entries in the user's own address book. Here's a possible set of directions for doing thisadapt this for your own situation: