One way to do determine distance between two or more Androids using gps is to use a CloudDB or FirebaseDB as shown in this simple example. The distance may be determined using any of several methods. This uses tools in the Map component to calculate the distance; alternatively an algorithm to calculate GC distance beween coordinates using the gps hardwarer in an Android.
Also search the comunity for several discussions using the search term ( uber ) for other idees.
