I used Hackmd in the past to share the draft of my book (1) and liked that people don't need to have an account to comment. Google Docs was no option as no markdown support and account required. The process worked well but I found Hackmd too expensive for just getting feedback. Stash looks promising for this use case.

(1) Written fully in Markdown in Obsidian at this point. I moved to Asciidoc since because of formatting. The early draft is still available on Hackmd though. Details in my bio.

We've used HackMD in my OSS project since Google Docs was failing us as far as version control goes. What's great is that you can sync a HackMD document to GitHub or similar, so you can collaborate on a document and then push it to a repo.