You are looking at documentation for an older release. Not what you want? See the current release documentation.
eXo projects use GIT and github.com for managing source code. Thus, to contribute to the projects, you need to install GIT and register a github.com account. See http://git-scm.com/docs to learn to use GIT.
Many eXo projects are public at the eXo Platform repository, so all GIT users have the Read access. To contribute to a project, you need to have the Write access.
For usage example, after installing GIT and Maven, you can build eXo Platform in some steps:
Check out the project: git clone git@github.com:exoplatform/platform-public-distributions
Go to the platform-public-distributions directory and build the project: mvn install
You will see the eXo Platform package in the plf-community-tomcat-standalone/target/ folder.
To write your own extension, see this sample project.