The first letter has to be any letter between A and X (Y and Z would have repeats). The second letter could be anything from B to Y except for the first letter... This was my way of figuring it out but it made it more confusing than anything else.
I understand it now... I was thinking of a number of different possible combinations rather than number of members. So initials ZDA for example would have counted since it is part of the ADZ group. This page was also helpful: