When you’re reading messages in Mail, you can automatically add someone’s email address to Address Book simply by typing Command-Shift-Y. And once you create a group address in Address Book, you can add all of the group members simply by type the group name in the To (or cc) field in Mail. Here’s how:
And you’re done. The next time you want to send email to the team, you won’t have to enter each name one at a time. Just start typing “Tennis Team” and let Mail fetch and fill in all the email addresses for you.