One possible course of action: email these 100 people explaining to them that while you have a good number of 'intent to buy', you can't launch the app in abse

One possible course of action: email these 100 people explaining to them that while you have a good number of 'intent to buy', you can't launch the app in absence of more buyer interest. Next, ask/request their help in terms of getting more signups. (As per Richard Bach's Cosmic Law,) Like poles attract i.e. people are likely to know others having common interest. If your app is useful enough, (hopefully it is,) the current 100 will help you get more leads. HTH