Android : Develop an app that finds other app users around you to interact with them

I'm trying to understand how to develop a very simple app but I only know a little of web developing (HTML5,JS,PHP,SQL) so I can't figure where to start. This is the general idea: users that open this app for the first time must register and have a public profile (and here's a big problem. how to handle registrations and store editable users data). When my app is running on a device, it should send it's location (via GPS+Wifi) to "somewhere" (to a database, I think?). Inside the app there will be a button to find other app users around you (let's say 100 meters). So there must "something" (literally a traditional server?) that processes a lot of data and that sends it's results to the device (maybe a JSON list of the devices around it, that my app could render inside the app). By clicking on "someone", the app will show his public profile and you should be able to interact with him in some way (just send an "HI" alert to his phone and waiting for his response. stupid things like that)

I'm used to simple static websites, so I can't realize "the bigger picture" of apps like that. Is it enough a traditional dedicated server with just a simple mysql DB and some PHP to handle read/write functions? Everytime each device would send his position, which will be written into the DB, and eventually read from it everytime a scan is needed, is that right? Does it scale? What if 10 millions of devices send/receive this kind of data at the same time? Wechat.com, for example, has a function that "finds" people around you using the same app, and manages millions of users at the same time without any problem. How do they do that? I use mysql databases everyday and they don't really scale very well. I don't think they are good for "real-time apps", are they? What about cloud services, REST APIs? Paid services like pubnub.com? Are they useful in this scenario? Thanks!