GitHub Pages "Normal" Setup for User & Organization Pages
Let’s say your GitHub username is “alice”. If you create a GitHub repository named alice.github.com, commit a file named index.html into the master branch, and push it to GitHub, then this file will be automatically published to http://alice.github.com/... The same works for organizations.
Read more here: http://pages.github.com/
However... the downside of this is that anyone that forks this repo won't get it as a GitHub Pages repo when they are working on it... because they have a different GitHub "username" (or "organisation name").
So the trick is to not use a master branch as the documentation tells you... rather, use a gh-pages branch, as you would for your other "Project Pages".