How to display number of followers on "subscribe by email" popup registration form?

I'm using the Subscribe by Email plugin and would like to customize the registration form to additionally display the current number of registered followers. The former subscription plugin I used had this a standard feature, but I'd obviously rather use the wpmudev version. I don't mind hacking the code a bit. Does anyone know what code to use to display the number of followers, and where to put it to display on the popup registration form?