If you frequently send email to the same group of people, you can create a group to make it easier. You can also use groups to organize your contacts by categories. You can send calendar invitations to a group and print the addresses in the group as a list, on mailing labels, and on envelopes.
You can also create a Smart Group that adds or removes contacts automatically based on rules you define. See Use Smart Groups.
Click the Add button 
, choose New Group, then enter a name for the group.
You can also Command-click individual contacts, then choose File > New Group From Selection.
Choose All Contacts, then select one or more contacts.
Drag contacts to a group.
You can also add a group inside another group. Drag a group to another group to create a subgroup.
Select the contact, then press the Option key.
The groups and folders containing the contact are highlighted.
You can send calendar invitations to a group and print the addresses in the group as a list, on mailing labels, and on envelopes.