Every single grade level has at least one standard that relates to collaboration or shared writing experiences. The beauty of these standards are that there are several ways to go about accomplishing this task.
*Note: This particular post will be continually updated based on the new collaboration tools that become available.
Some collaboration tools require an email, while others don't. Therefore, I'll be segmenting this particular post based on the need for an email address or just a username/password combo.
The first tool is the tried and true GoogleDocs. While you do need an account in order to set up a GoogleDoc, you do not need an email to edit one. Check out this GoogleDoc, and add your own text to it without signing in.
When setting up your own collaborative writing GoogleDoc, just be sure to choose "Anyone with a Link" and "can edit".