- Glympse on the iPhone and Android works well. It's the most real time with frequent position updates but also the most battery intensive. Free.
- If you have a Garmin they have a Livetrack option which generally works well and provides updates every 30 seconds or so. Much easier on the battery than Glympse. Free other than the Garmin cost.
- Strava has a 'Beacon' app. Used this once but don't normally have Strava running on my phone. Believe this comes with the paid premium Strava service - $10.99/mo.

None of the above apps are 100% reliable so you'll want to set that expectation ahead of time to avoid unnecessary worrying.