Get a heads up when you cross paths with a friend
Quick answer
To get notified when you cross paths with a friend, save where they live and let your phone compare your position to that spot locally, alerting you only when you get close.
Crossing paths with a friend by accident is rare in a big city, but it stops being random if your phone knows where they live. Save a friend's place to a private list. As you move through your day, the app measures the gap between you and that saved spot. When the gap closes, you get a quiet alert. No live map of either person, no shared trail, just a nudge that says someone is around the corner.
In one sentence
Parages pings you when you pass near a friend. It tracks nobody.