Markdown and Git should be very accessible to a non-technical audience and I've had good success with both.
Here's my favorite SAAS offering in the space - https://www.gitbook.com/
Here's my favorite SAAS offering in the space - https://www.gitbook.com/
All of those tools that look like Microsoft Word (have the B, I, U buttons) have something wrong with their underlying conceptual model. (You can tell from the way the selection behaves as if it was possessed by the devil.)
That conceptual mismatch is annoying for a single-user editor but it makes a multiple-user editor a complete mess.