Abstract: The statements of Main~Theorem~1.1 and Theorem~2.1 of the author's paper [\emph{Trans.\ Amer.\ Math.\ Soc.}\ {\bf 345} (1994) 577--594] should assume that $\Gamma $~is discrete and $G$~is connected. (Cors.~1.3, 5.6, and~5.8 are affected similarly.) These restrictions can be removed if the conclusions of the results are weakened to allow for the possible existence of transitive, proper subgroups of~$G$. In this form, the results can be extended to the setting where $G$ is a product of real and $p$-adic Lie groups.