on Z Visa, which legal CN authority need to issue invitation letter to invite my parents for S2

Hi,

I've been working in Suzhou, China for 3 years and want to call my parents on S2 type here.
I've prepared an invitation letter myself including all the details as you mentioned in this article.
I've also been told that I need an additional invitation letter from some Chinese legal authority. However I'm totally lost here; which legal authority? Should it be my employer (that is a foreign company in China) or is it some specific government office?

Thanks,
Zam