When I used to use Atom, there was a great collaborative writing tool called Teletype. It allowed multiple people to be editing the same files at the same time. I've been working on a book lately and wanted to get another one in progress with a group of authors, so I tried to find something that would duplicate Teletype's functionality iThonically.
A few weeks ago, I happened onto MWeb and everything changed. MWeb has a great preview, a good library that integrates well with its desktop version, and allows easy editing of files in external folders, including the git repository that hosts this site.