Raceday.me

And this is how it works:

1) set up a race
Signup for a free raceday account. Upload the track of your upcoming race and set your desired finishing time.

2) share the link
Your friends can follow your race progress live from anywhere in the world. No app required.

3) start the app
Start the app just before the race. Low-power mode ensures not to drain your battery.

Go to https://raceday.me/demo to see a demo run.