I would suggest launching in certain geographical locations ONLY.

Kind of like how Uber does it.

An app like you're describing is pointless if it's value comes from location based services.

Push one small geographical locations through marketing tactics that will bring people to your app download page. Think of localized contest in which the prizes have something to do with the location (major league baseball tickets, for example).

If someone attempts joining from a different location, inform them you're launching soon and keep their emails on file.

This then tells you where the most demand comes from, and give you a good idea of the next location to "open".

This also allows you to test what is working and what isn't because your app will be used by people as if it were already popular all over the world.