Or simply drag and drop.
First open Mail, and create a new blank message.
Then open Address Book, View > Groups. On the left, scroll up to the top, and choose "All Contacts". On the right, select a contact, and then Edit > Select All. Then drag all of these contacts into the "To:" field (or CC, or better yet, BCC) in the blank email. (BCC will relieve your recipients the burden of looking at the email address of every one else you're sending to and exposing the entire contents of your address book to each one of your recipients.) In Mail, see View > BCC address field.
Warning - some email systems will consider you to be a spammer if you send an email to more than a certain number of recipients, so your mileage may vary. If you send to less than 25 recipients per outgoing email, you're probably safe.