It's not possible, and without a screenshot we can't tell what it is you saw. My guess is the guy was paragon 68. If there was a way to get people into a T13 game underleveled that would be abused like hell on season start, so simply: not possible.
Well, then it's a recent exploit. Would still love to see a screenshot. Not saying it's not true - but it's hard to believe since this should obviously not be possible, so if you find such a thing take a screenshot and send it to Blizzard/post it in their bug forums:
I created a new hero, made a public expert game in adventure, tag rift and leveled to lvl 11, then since there was nobody who joined, i quitted and made started another public game in hard mode, but instead of joining adventure rift hard, i joined adventure bountie T13 and there were several people at parangon 500+ in the game. Sadly i did not take a screenshot, but i think it is a bug.