You can add and edit your groups by clicking on the List Manager on the left-hand side of the screen and the clicking on the Groups Tab.

In order to add a group, click the green plus symbol at the top of the list. A pop-up will prompt you for the name of the group. Enter it, and press the OK button. This will add a new group to your Group list.  To edit or delete a group click on the group name.

More information can be found in the groups section.