Here some of the web tools to encourage and foster collaborative writing :
1- Google Docs
This my number one tool for writing collaboratively. Google docs has all the features teachers need in order to hep students write and work in teams. Check out this guide to learn more about Google Docs as a writing tool.
This is another awesome web tool that allows people to work on the same document simultaneously. It does resemble Google Docs and it also does not require any software download.
This is a very simple writing editor that has great collaborative features. Students can invite their peers to edit their writings and share feedback with each other