Mobile advertising to target users with high precision and through multiple funnels like handsets, OS , handset models , OS version , geo location, demographics, genre , content etc . Define your user segment based on these parameters . Like to target an Affulent user who is stock market savy... Run a campaign targeting iPhone 5 and iPads 2 and above on apps like Bloomberg , financial times, WSJ etc . Thus you can create your targeting segment and target them by running a campaign through good ad network or directly with publishers.
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.