The current (May 2016) released version of the server is 1.3.3 (zip or tar.gz). This is the version used in the 2016 Power TAC tournament. See the participant's wiki for detailed setup instructions and for information about setting up a development snapshot of the server.
Software development is hosted at github. There you will find a set of repositories for the server, the agent framework, the admin web application, the game visualizer, and other major components of the Power TAC simulation infrastructure. You will also find our issue-tracking system, and the developer's wiki with a wealth of information on the design of the server and its evolution.
Releases are announced on the developer's mailing list at nabble.com, and release notes are posted on the participant's wiki, and release artifacts are hosted on github. Server release packages are quite small, generally no more than about 5kb - just a README file, a sample configuration file, and a Maven pom.xml file. Maven will download the modules and all their dependencies when you first run it.
The core development team includes groups at the University of Minnesota in Minneapolis, Erasmus University in Rotterdam, Aristotle University in Thessaloniki, and the University of Zagreb. If you are interested in becoming a developer, you may want to start by joining the developer's mailing list. Then browse through the wiki and the code on github, set up a development environment, understand our project development policies, and volunteer for an area that interests you. Of course, you are always free to fork a module or two, make changes or additions, and issue pull requests to have your changes considered for inclusion in the server.