I've raised this in the past ( https://moodle.org/mod/forum/discuss.php?d=223674 ) - and as it's returned to haunt me; I wonder if it would be possible in future releases to have a toggle in the set up, so that if it's been set at site level to allow one member of a group to book a slot for the whole group, when you're setting up a particular scheduler, to have an on/off "Allow 1 group member to schedule the whole group".
That would seem to allow for both options.