Grift Score - this is a simple calculation where it is (rift level * 900) + remaining secs in last rift. Your overall player profile grift score (found at top of your profile) is the best score of any class in 2,3 and 4 group categories. Solo grift score is not used due to class imbalances. Clans do not use Grift scores, instead they use the worldwide ranking scores for all grift categories. The GR clan score is basically your clans best worldwide rank added together; so the lower the number the better.
DPS and Power are fun metrics for those who want to deck out your heroes and then compare them against other players in the clan or site. Power is similar to DPS but uses other metrics like recovery, ehp, paragons and even gem levels.Read this thread for some more information about clans