From the parse docs:
// User's location
PFGeoPoint *userGeoPoint = userObject[@"currentLocation"];
// Create a query for places
PFQuery *query = [PFQuery queryWithClassName:@"_User"];
// Interested in locations near user.
[query whereKey:@"location" nearGeoPoint:userGeoPoint];
// Limit what could be a lot of points.
query.limit = 10;
// Final list of objects
placesObjects = [query findObjects];
placesObjects will be an array of objects ordered by distance (nearest to farthest) from userGeoPoint.