Google Drive/Docs is great at working everywhere and offering instantaneous collaboration. Drive is not so great at letting you review changes and track specific versions of your document. Enter Draft, a writing web app that works with Drive, Dropbox, Evernote and Markdown.
On its own, Draft is a very nice and minimalist place to write. What sets Draft apart is its collaborative editing powers. You can invite people to pick at your words, but Draft tracks the changes they make and asks you to accept or reject them, individually, while showing the two different versions of the document in side-by-side columns. This way, you can invite more than one person to review your work and keep track of who made which suggestion.
You also mark drafts as you write, making it easy to get back to ideas you may have discarded as you went along. That is, suffice to say, nicer than running through Drive's colour-coded overwriting and immense list of revisions by username.
Draft packs in a lot of other features without intruding on the writing-focused interface. You can bring in professional copyeditors for important work, write and convert from Markdown, import and export documents from all the popular cloud services, and -- my favourite -- install a Draft Chrome extension that lets you click in any text field on the web to open a new Draft, then click again to paste your work back into that field. In fact, that's how I wrote this Lifehacker post.
Draft is free to use, and you can sign in using your Google account.