Does Drupal have a collaborative editing module? What I would like to offer my users is the ability to propose a writing project to a group of people (users and non users) and the people who want to join the writing project are able to edit the same node together in separate locations.
The answer from knowledges33ker is a good start. 

However the workbench module - https://drupal.org/project/workbench
 - might also help with this and add further functionality.

Particularly in that it offers good tracking of edits that have been made to content and you can revert content to previous revisions etc.

With the user reference field set up, you could also, if you wanted, use the Rules module - https://drupal.org/project/rules - to fire emails or notifications to users when the content is edited/updated.