Travel on F visa to other city not mentioned in invitation letter?

Am I able to visit other cities like Shanghai and Beijing if I were to arrive/depart from Guangzhou but these cities are not mentioned in the invitation letter? How restricted am I from travel with F when staying with a friend who has provided invitation letter? If I would like to see other friends, must they visit me, or can I visit them when they're available and notify some authority about my travel to another city that was not mentioned in the invitation letter?