The way we've always run group rides:
Leader knows the route, almost always planned the route themselves, and doesn't need a map/GPS to follow it. If the Leader loses sight of the TWO riders behind, they pull over & wait to re-acquire. Leader usually has comms equipment linked to mobile phone, and EVERYONE on the ride has the leaders phone number. No-one overtakes the Leader, unless they wish to depart the convoy.
Bike immediately behind the leader is the marker for that junction. They stop & point the way for everyone, until Tail End Charlie rolls up & flashes lights to signal freedom.
Tail End Charlie, also knows the route implicitly, and has probably ridden it with the Leader before the group. TEC passes absolutely no-one, with the sole exception being to block traffic to let the group out in one go, if safe to do that.
Following the rules, the only way to run out of markers, is if everyone is at a junction pointing the way, and TEC has had a breakdown. In which case, Leader loses sight of the 2 bikes behind, pulls over and waits. Some minutes go by, and Leader calls TEC "hey, what's up? Where are you?"