Clusters not forming is a typical error for beginners. Most often people start by trying to create a cluster in their own environments, e.g. server or laptop. So it'd make sense to add a java option that binds JGroups to 127.0.0.1.
Users can always modify the variable to their node's external interface when trying to create clusters across multiple machines.
Without this option, users have to know or look up the documentation to figure out how to do it.