S1 Visa from Pakistan

I would like to inquire about S1 type for my family. I am currently working in PRC and have Resident Permit for Foreigners. I want to Invite my family to stay with me as long as I am staying in PRC.

I have seen on China embassy (Pakistan) website the documents required for S1 type.
I do not understand one document, can you guide me please;
1). Invitation letter of Duly Authorized Entity. (Please guide me who will give this letter)