What is the best way to isolate a niche group to target as potential users of our iphone app?

I am sorry - I think it should work backwards: you identify your end users and their problems and then design the product and get back to them.... Otherwise it is an old problem on inventors making "solutions in search of the problem".

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.

The best ways to isolate a niche group to target as potential users of our iPhone app are as follows:
1. Identify your strengths and interests.
The best niche marketing strategies play into your brand’s unique strengths and perspectives. Also consider the areas that you enjoy working in and the people you like working with.
2. Do industry research.
Once you have an idea about the type of niche marketing you want to do, validate that it is a reasonable idea. Also look to see if any openings in your target market may have been missed and if there is legitimate demand in the vertical. Because Amazon is such a massive online retailer, it is a great place to get inspiration for product categories you may not have thought of. Spend some time browsing to see how to drill down into popular product categories.
3. Get to know your ideal customer.
Another way to gain insight and spark inspiration for niche marketing is to look closely at your target audience and identify what they really want and need. To research your ideal audience, use Alexa’s Audience Overlap Tool. Enter your site or a site that has an audience you would like to reach. From here, you can look for trends that tell you what else the audience might be interested in. You can identify ways to focus in on your ideal customer’s needs and find opportunities to market what you offer.
4. Choose, test, adjust, and repeat.
Like most marketing strategies, you cannot just set up a niche marketing campaign and assume it will achieve the results you want. You may find that your first idea for niche marketing did not work, but that a simple tweak could hit a sweet spot that draws in audiences and leads to lifelong customers. Perhaps a full boutique shop for yoga enthusiasts did not catch attention, but you noticed more than half of the shoppers you had bought artwork.
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath