How would this work (cell phone contract help)?

My aunt and uncle have invited me to their phone plan. To rid myself from the cheap t mobile phone I have. They have att. I plan on getting the Motorola Atrix. Soo. how do I go about this. Im assuming it would be the 2year contract price. But when it comes to data packing and all that. Would it be like single persons phone bill combined with them. They have unlimited texting and 1400 mins.