How do I get status of people near me

I whant to do a app that uses the geolocalization, but i also need to know the people near me that also uses the same app.

does anyone have already used this?

Guys i really need at least a info were i can start.

I was thinking about seting up a mysql server and a app that gets my position and send it to the mysql server, so i do a request of people in a radius of 20 km of me, i need a list of those people.

I dont know if that is the start.