Hi there,
I would have a few questions before making any suggestions, but in principle, there are 3 stages I would work in for this project.
1) to evaluate the app and the users you have already, where they come from, what they are interested in etc
2) come up with creative ways to engage them further with the content you are looking to promote
3) find ways to take this further and ensure the best is made of the opportunity for you to have success.
I've been working in digital marketing for almost 20 years, solving problems like this for brands and businesses, startups, events and organisations including the likes of Coca Cola, Microsoft, Apple, Subway, Xbox, Sony, L'Oreal, Garnier, Google, Louis Vuitton and Tiffany - in fact over 200 brands and businesses i've worked with over this time.
I've also been an independent consultant for Facebook, TFL London, Google & Twitter on a number of assignments, including social media activations for London 2012.