Sysmarine founder. Twelve years of experience in tech companies. Specialities: mobile and embedded systems.
You should try to engage people using social networks, it is easier to spread than email. The conversion rate on emails are low but is still a valid tool for that. Send and email with a simple and objective message that will make people want to try.
The best way to have feedback from users is to watch them use the app. You should put them on the hands of everybody that you can and without any instrucions and just watch, don´t even say that the app is yours. Try to do it a lot.
If you want feedback from others, you can include the feedback form inside the app and suggest users to answer occasionally.
I would also strongly recommend to use a tool as Flurry Analytics. Is the best way to get data from how is the use of the application.
Pay attention to those data and be open to change your app a lot, you may need more features or cut some off to make it easier to use.
If you need more help please contact me.