Use the options on the Preferences>Address Book folder as follows:
Settings
Select Add new contacts to “Emailed Contacts” to automatically add addresses to your address book when you send email. If this is enabled, addresses are saved to your Email Contacts folder.
Initially search the Global Address List when using the contact picker can be enabled if you frequently need to find addresses that are in your company's address directory but not in your address book.
Autocomplete
Enable Include addresses in the Global Address List if you want to be able to see addresses from the company's address book (Global Address List) when you type an email address.
Enable Include addresses in shared address books if you want to be able to see addresses in address books that are shared with you when you type an email address.
Note that if you enable either of the two above features, when you type in an address, you may get many false addresses listed if these address lists are very large.
When Perform autocomplete when a comma is typed is enabled, autocomplete recognizes the comma as a "command" to make a quick completion of the name and automatically add it to the recipient field.
When Don't show contact group if one of its members matches is enabled, when autocomplete is used to enter user addresses, contact group lists that the users are in are not displayed in the auto-complete list.