Given your motivations and comments, I would recommend that you have a whatsapp like subscription in place. One can get around paying for the subcription by inviting friends. For best results, make it easy enough to select all for the invite [you will soon realize that you'll get most of your users from such people who don't mind inviting a friend to an app they like a lot]. At some point in the next 1-2 years, you would likely get to 1 million downloads!