How to get nearby alerts without sharing your location
Quick answer
You can get alerted without sharing your location: the proximity check runs entirely on your phone and only fires a local notification, so your position never leaves the device.
Sharing your location and getting useful alerts are two different things, even though most apps bundle them together. The trick is to keep the computation on your phone. Store where friends live locally, then let the device compare your position to those points without uploading anything. When you land near a friend, a notification fires. You get the alert; the server gets nothing, because your location never leaves the phone.
In one sentence
Parages pings you when you pass near a friend. It tracks nobody.