To test whether JavaGroups works okay on your machine, run
java org.javagroups.demos.Draw -trace. twice.
2 whiteboard windows should appear. If you started them simultaneously, they should initially show a membership of 1 in their title bars. After some time, both windows should show 2. This means that the two instances found each other and formed a group.
When drawing in one window, the second instance should also be updated. As the default group transport uses IP multicast, make sure that - if you want start the 2 instances in different subnets - IP multicast is enabled. If this is not the case, the 2 instances won't 'find' each other and the sample won't work.
You can change the properties of the demo to for example use a different transport if multicast doesn't work (it should always work on the same machine). Please consult the documentation to see how to do this.