You have a lot of options when it comes to online writing programs, but Editorially manages to combine some of the best features of all of them. It combines a minimalist writing environment, powerful version control and extensive collaboration features in one easy-to-use program.
Editorially uses Markdown for its formatting, but it otherwise works like any old plain text editor. Type your words and Editorially saves them in multiple versions as you go. Then, when you're ready to share, you can invite friends and colleagues to make notes and edits. These notes aren't just little snippets you'd find in Word or Google Docs; they're full, easy-to-read conversations that enable a group to actually discuss the text.
Editorially is currently in open beta right now, and it's free to sign up.