I've seen the idea of organically growing communities indirectly and directly mentioned in various threads when people discuss which communities they'd like to see on instances, and in a different way in response to community creation announcements. Despite this, and some inconsistent efforts on my own part, I've not seen too many others appear to be trying to put this in action.
I think any of the open-ended chat/conversation/general communities are a good place to start with this, but I've found it tricky to work out what to post to them. I suspect that may be somewhat similar for others, but being in the boat with'em, I'm not sure how to help.
Regardless, I think these communities make more sense for people to find those that share their interests to then start their own communities vs. starting communities before knowing if anyone else is interested.
What do you think, and what do you think would help people feel comfortable posting in these broader communities?
I think I see where you're coming from on this, and to clarify I'm not thinking so much of people developing common interests and relating individually as you describe. Instead I'm thinking in terms of people posting about an interest in a broader community, like Schizo describes with a hypothetical Hobbies community, to gauge interest and carry on for awhile until it's pretty clear they'd benefit from creating their own community.
A classic example is like the TV or Games communities that eventually draw enough people interested in the same specific shows/games that they then create sub-communities dedicated to those specific shows/games.