I didn't know anybody here before I joined. (I have been here for a few years, and I still don't know anybody here.) How would a person like me get invited or vouched?
If the discussion is related to a public project, like in the examples in this comment: https://news.ycombinator.com/item?id=47303604
...you can use existing communication channels (website, readme on github) to ask people for an invite to participate in it.