We used the free Life360 app on the CMA ride last weekend. Only 2 of us tried it, but it did show where 1 part of the group was when the other part of the group was having a delay. It works on Android and iPhone. It seems reasonable that each riding group could have it's own circle, and could see each others location and see each others "check in" comments. Cell coverage is necessary, and we had cell coverage most of the time I looked at my phone (Verizon, iPhone 7) during the ride.
That's just 1 option! Jeff