"In addition, by keeping the rank discrepancy between party members to a minimum, players can enjoy an increase in the amount of skill points awarded from combat. The smaller the range in party member rank, the greater the effect of this bonus will be. The effect of the bonus also grows in proportion to the total number of party members. The largest bonus possible, therefore, would be a party of eight members, all of whom are the same rank. All players ranking up Disciplines of War and Magic classes are encouraged to take part in parties and make the most of these bonuses."

Source: http://lodestone.finalfantasyxiv.com...de/comm02.html