Another way to divide players into groups inside the prediction game are groupings. As for the team rankings (more at Team rankings in prediction games), groupings are a new ranking on top of the individual ranking.
Create groupings as the administrator
With the "Groupings" feature, groups are not being made by the players but by the admin. A grouping should hence be chosen when groups are already made. For instance divisions in a company. Or specific regions of newspaper readers. Or men vs women or young vs old prediction against each other.
Group choice at registration to the prediction game
All players can choose their group when registrating to the prediction game. The administrator can later edit these choices if needed.