To modify a group within your address book, select the Modify Group Lists button in the Address Book field. From here, you can pick the group you wish to modify and select the action that you would like to perform. You are able to create, rename, and delete groups as well as add and remove users from groups.
Example: To add a user to a group, select the radio button for the group and select Add To Group. You will then be taken to a list of all eligible numbers inside of your address book to choose from. Select the names you would like to add and hit the Add To Group button. At that point, the users that you have selected have now been added to the desired group.