Looks like you got a lot of good responses. I was going to mention the "GPS phone tracker Pro" app but the Life 360 looks like it does about the same thing. I've never used one of those apps myself but I've heard they work pretty good.

Now if you are going to use one of those in addition to a typical navigation app ( such as RWGPS app ) at the same time, I have no idea if both can work at the same time without some kind of problem. However since the tracker apps are basically designed to run in the background I wouldn't think there would be a problem with both running at the same time. Of course if you have to turn off your navigation app to check the position of the other rider on the screen that may or may not be a problem depending on what navigation app you are running.

Just keep in mind with two apps using GPS function of the phone at the same time I would tend to think that the phone is going to use twice as much battery power. Of course if you've got a large battery or a battery back-up I wouldn't think it to be a problem.

Fifty people! Goodness, that's a lot of people. That almost classifies as an "Event" rather than a standard group ride. With that many people you might be better off having someone follow with a support vehicle just in case some Bozo has a major mechanical or a medical emergency.