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.
If you cannot or do not want to install Etherpad Lite, you could use the Piratepad Drupal module, which accesses the Etherpad Lite installation on piratepad.net or similar sites. (Of this, there is also an unpublished D7 port.)
(Source: drupal.stackexchange.com answer #53188)
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.
You could install the references module, then add a user reference field to the content type. Then, install the Node Access User Reference Module (https://drupal.org/project/nodeaccess_userreference). You can set your configuration so that users of a given role can add another user(s) to the node and you can specify what permissions those editors can have (e.g. edit but not delete the node, etc). This gives you (and/or your users) more control over when specific people get to "join" a writing project with editing permissions.
You could also try the drupal wiki install profile: https://drupal.org/project/drupal_wiki