If you are sending out a group email and want to use a group you have made in Address Book, you can just drag over the group icon from Address Book into the Mail address field. An even easier way is to just type the Group Name in the address field. Mail will recognise it as a collection of email addresses. For instance I use a smart group called Dublin to collect all contacts with an address in the city. Simply typing "Dublin" into the address field will address the mail to all the contacts within that group.
Tested under 10.4