I need detail instruction on writing invitation letter for my family willing to visit me (S2 visa)

Hello! I am studying in China with an X1 status and my family wants to visit me from Bangladesh with S2 . For applying for S2 they need an invitation letter from me. I want to know who should this letter address to and whether I need to write 3 different letters for the 3 different family members or should I write one letter mentioning the complete information about all of them?

For more clarification, let me ask, if my would suffice. If not what should be done. The international students' office in my school says they do not issue such letters.