Question

What important group did selim bring under ottoman control?

Answer 1

I assume you are speaking of Selim I. Selim conquered 2 groups that would have been very important. He conquered the Safavids in modern day Iran. This would was key for the access to the trade routes and the taxes that one could gain from the amount of the goods that passed through. They also gained to Egypt and the fertile growing fields when they conquered the Mamluks. Egypt also gave them access to the ports and again through trade.  

Answer 2

Sultan Selim brought a very important group the safavids. If you didn't know safavids were shia muslims and the ottoman empire (sultan selim) they were sunni muslims. Sultan Suleiman the Magnificent (sultan selims father) had a big rivalry with the safavids. Surprisingly his son Sultan Selim who was not expected to be a sultan after him became friends with the safavids. The safavids helped Sultan Selim kill his own brother şehzade bayezid and his children so he can be sultan. And that's how they became friends.