Both the Account Contacts page and the Contacts Widget (when in address book view) have the ability to set up groups. Click the Create a Group button to make one.
To add contacts to your group:
- Check the box next to them to select them
- Click the Add to Group button and you will be presented with a list of your groups
- Check the box next to the groups you would like to add the contacts to
- Click Add to Selected Group(s)
To remove contacts from a group, click on the contact and click Remove next to a group listed under the contact. Alternatively, when in a group you can select multiple contacts and click the Remove from Group button.