I know that u meant that, thats why i said "Not overall, as it make no sense. Also not by month" For overall you have your topsite member count. For month: i dont know how to modify the script that way, someone else may know.
Thats why i showed the alternative.
Sorry if it not fits your needs, but getting what you want might get complicated, as Mod creators get nothing for their work and they rare these days.
It should work in every of your skin html files. As experience showed me, it would best fit into table_wrapper.html ( to only show it on the ranking sites, nowhere else ) or in wrapper.html maybe in the sidebar or so