They have a family plan of sorts for data. "Family Data Unlimited", which can only be used with feature phones (non-smartphones), provides you a $5/month discount per line on unlimited data. There is no such analog with smartphones, which now require either a 200MB or 2GB data plan per line.
True family data plans will only come with AT&T killing off unlimited data plans completely. Given their new pricing structure on their smartphone data plans, I don't see them offering such a plan anytime soon. And I wouldn't count on rollover being a part of that equation. Data has always been, and continues to be, "use it or lose it".