Founder of Touco Direct, NoCode Development Advocate, Startup Mentor, Serial Entrepreneur, Wordpress Developer
First and foremost, I would challenge you to test the user experience and prove the concept of your product before building a mobile app. Mobile app development is not cheap and if you get the feature set wrong from a user standpoint, you will have spent money to develop blindly. Can you build a similar experience that provides value in a web application instead? You probably can. Even a few hundred users can get you the information you need to know almost exactly what the mobile app needs. There are mobile first websites that were around for years before they built the app just to make sure the experience was on par with what they expected. Test, test, test, and test again before jumping out there.
This is a very hard question because the experience of each entrepreneur will be different based on what they are seeking to gain as well as what they contribute. Develop a list for yourself of what you seek from a mastermind and then start looking. House of Genius is one I have worked with that is hugely collaborative, though not officially labeled a mastermind group. Meetup has lot of groups with specific targets but again, it's best if you know what you seek to gain, also how you wish to contribute, before searching for a mastermind.