What I'm going to offer something that is a bit tricky and you need to use it with a lot of caution. It will also require some development.
One of the ways of growing fast and get additional users is by having current/ new subscribers to send their contact list a notification that they have joined your app and offer these contacts to join as well. You can add a screen after the first installation suggesting to send an invitation to certain friends (based on location. e-mail address/ Facebook criteria etc.), but you must make sure it is an opt-in option so you won't get your app spamming the current users contact list. This way you generate more users from current users sending them an invitation to try your app.
Does that make sense? I will be happy to elaborate if needed, just send me a message.
Good luck