Smartphone app that alerts when a rider is left behind
I don't believe there's such an app, so here's an app-assisted approach.
It doesn't alert but both clubs I ride with have used WhatsApp location sharing to meet up on the road. Other location-sharing tools exist too. If you use a back marker (a role I've often taken in the slower of my clubs, being fairly strong and well equipped, with maps and a loud voice), their job is to stay with the slowest rider and they could share their location with the leader. With big groups, designating a back marker (or pair of back markers) is worthwhile. They should have the route and good lights if darkness is an issue.
You could run location sharing all the time, or you could rely on starting it if you're dropped - this would be more suited to mechanicals when you're stopping anyway, as I can't really recommend that much fiddling with your phone while riding, even with it clamped to the bars. Setting up a group for the ride (or series of regular rides) would be necessary.
In fact the messaging alone is useful - I've received a few &quot;you ok ChrisH?&quot; messages when I've been dropped from a faster group for reasons including cramp, a last minute reroute, and of course simple slowness. These are best if the leader says where the group is waiting (assuming they are) as you don't want to stop to reply, then come round a bend and see them.
Location sharing relies on a data signal, which is fine for local road rides, or my nearest trail centre, but not much use for wilderness cross-country. Without it there may be push-to-talk radio apps that use a WiFi signal. That should give you enough range. Whether any can display location is another matter.
Update October 2020: If the group (or more likely a leader and back-marker) all use IpBike on their phones as bike computers, IpPeloton (under development) shares the riders' stats across the group - but only if they're in range.  This would work for a tight group, but not for a loose one.  You'd need to test what's shown when someone drops off, and how far away they have to get. The inherent range limits of this technology mean it's probably not the way to go; either a proper data connection or a WiFi network between riders would be necessary.
Further evidence for the unsuitability of Bluetooth Low Energy is given in this  preprint which (in the context of contact tracing) discusses proximity sensing using Bluetooth beacon signals and only being able to the read signal when &lt;10m from the other device.