I don't know of any software designed for that scenario, nor, as a fairly frequent ride leader, would I see value in having it. The group tends to ride at varying speeds based on the preference of the participants. Having someone stay with the slowest group ensures that no one, especially any inexperienced rider, gets left behind. I don't see the point in trying to maintain a minimum or maximum separation between the lead group and the sweep since doing so would either force the lead group to slow down from their preferred pace or force the slowest riders to speed up (which might be beyond their abilities). If someone in the middle feels the need or desire for more nearby riders they have the option to slow down and wait for others to catch up - no need for any electronic aid to do so.