2- you may forward the position of each users to a central server and perform the proximity matches there, and send back the list of close users. That means that IP connectivity is required (GSM or Wifi).
In any case, you get precise geolocation only when you have wifi networks visible, so you may as well require that the user enable wifi.
Once you have wifi enabled, it is possible to use it to scan close users directly, without using geolocation, by mere physical closeness.
You can also do that with bluetooth for closer distances.
So, how you would go about this would depend on what level of privacy and what kind of application you want to do that for.
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.
With PostGIS you can use ST_DWithin to find results in a certain radius from a longitude and latitude point which is retrieved from the cell phone in iOS.