There is something to be said for familiarity. There has been more than one occasion when I have tried something new, but couldn’t get my team on board. Since I work with folks from many different organizations, google docs is one of the top 2 or 3 most familiar writing apps out there.

Simple, beautiful, with great tools for adding structure.

Not as cool as the cool kids, but it works and it's helpful when collaborating with folks outside the org as well (pretty much everyone has access to Google Docs in some way).

I know it doesn't support markup directly - but it ticks all the other boxes in your requirements list. There's also a Google Docs to Markup extension for the final step of exporting: https://gsuite.google.com/marketplace/app/docs_to_markdown/700168918607

We also use Slite and Basecamp, but in terms of collaboration features, Google Docs works the best. Doesn't support markdown, but you can use keyboard shortcuts for headings, bold, ordered and unordered lists, etc.

I guess this depends on whether you're looking for a free or premium solution. Full Disclosure: I've never used paid services for this use case. Google Docs has great collaborative functionality. The comments/assign feature is incredibly useful and being able to review historical changes saves a lot of time. Plus it's included in Google's business suite, which I'm assuming 99.99% of companies use for corporate matters.

Draft is a writing app that's a bit more like the individual focused writing apps like iA Writer and Ulysses, only its a web app with an incredibly simple interface. If you want to write without distraction, it's great.

Then, when you need edits, you can edit words and sentences—and include a comment to say why you made the change, Google Docs style. You can share the doc with others to get them to edit it. And you can preview document versions side-by-side with a diff view that shows what changed.

And despite being a small, indie app, it's still alive and maintained—and used daily by its founder, @natekontny.

I still use Google Docs most often for collaborative writing, because it's something almost everyone uses and understands. But if your writing team is looking for a more writing-focused collaboration app, it's worth considering.