Here's what I know about Medium Facebook groups:

https://medium.com/ask-an-editor/are-you-thinking-of-increasing-your-readership-by-joining-a-medium-facebook-group-34936d0fb985?sk=3ecdc88bb88168db358c9850a7258147

I can't help you when it comes to fiction. I don't read or write fiction on Medium so I don't know much about it.