i don't know what you mean.
also, a note on terminology: groups are named and joined with a Group Specification ("groupspec"). the GroupSpecifier class helps you build groupspecs.
you must be explicitly told about a groupspec (or the GroupSpecifier parameters used to build it), or be able to guess it. there is no way to passively detect groupspecs that are currently in use.
Well can a Net connection can send data to a groupdSpecifier?