Ask HN: Where can I find non-dev professional communities and groups?

For a variety of reasons, it seems as if developers have the highest online presence out of all professions. Whilst I do enjoy interacting with my peers, the brief discussion I've ever had with other professionals have always been valuable, and I crave more.

With that said, where can I find them? Where is the community for marketers, business analyst, and accountants? For lawyers, salespeople, and PR? Do they just not exist online?