In my opinion, if you have a question regarding a spec you are writing and you want to ask someone to kindly check what you could be doing wrong, you should push your spec to a GitHub repo - even a throwaway one - and provide a link. That way people can collaborate together by providing comments, creating pull requests etc.
I've always viewed this group as a discussion forum to ask questions and to get help on tooling and the TLA+ language. I don't think any Google Group is a good place to exchange code/specifications.
A Slack workspace for example, would help to avoid sharing code via GitHub, but I am not entirely convinced Slack would be a good fit for the TLA+ community.