JGroups ships as a single JAR file: jgroups-x.y.z.jar. Simply add it to your classpath and start coding !
The JGroups version can be printed with java -jar jgroups-x.y.z.jar.
There's a simple draw demo, start 2 or more instances: java -cp jgroups-x.y.z.jar org.jgroups.demos.Draw, and see if
the members find each other.
The sources for the core classes are in jgroups-sources.jar.
If you want the complete sources, including unit tests, go to http://www.github.com/belaban/jgroups.
JGroups is hosted at www.jgroups.org.
For questions use either the dev or users mailing list.