Facebook groups are currently receiving an update. The refresh serves dual duty, first of all making a group page much more visual, similar to the new Timeline. The second goal is to surface group members, particularly useful in larger groups where members can often get buried.
There doesn’t seem to be much of a method to the selection of group members that take on of the 8 top positions, other than simply randomized. There’s also no option for group admins to select or highlight users in the banner. The update also allows for easier access to Events, photos and documents inside each group.
As with most Facebook updates, these are being rolled out, so if you don’t see the change yet, try again tomorrow.