I am not sure if you are asking how to find a group that may be interested in your app or how to reach your target market. If it's the former then I agree with Eugene. But if it is the latter then there are many advertising options that allow you target a specific market. For our apps at theAppVillage.com we have had success in using facebook mobile advertising, using their advanced targeting features. If you would like to some more insight from our experience at AV please feel free to give me a call.