I have a 14' x 14' platform I am covering. Would I need to seal the seams, or do they not shift? We do workouts such as Insanity and maybe buying some weights.

Our Regrind Rolls are very heavy and wouldn't move while doing Insanity or other high impact workouts, so there is no need to seal or glue the seams. If there was still some concern that you may move the rolls, you could place some double-sided carpet tape down the seams and around the rolls' perimeter.