Here is a quick and easy way.Index all the user locations into a elastisearch instance and as soon as a user opens up the app , fire a near query on the elastisearch with users current location as input and get all the other users within a specified distance as json from the elasti instance.

This should help you http://www.elasticsearch.org/guide/en/elasticsearch/referenc....

The second option is to use mongodb for the same, but the first solution is much easier and faster to implement.