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.