Etherpad

This is a well known system for real time collaborative editing of documents. Just log in, choose a document title and then edit. Different users will appear in different colours, and can also chat in the sidebar. This is installed as a private system in which only users on your LibreServer server will be able to create and edit documents, so it's not open to any random users on the internet.

Ports

Forward port 80 and 443 from your internet router