New riders are doomed either way if they cant detemine their own pace. In front there is a pressure that "these guys behind me want to go faster, MUST GO FASTER". They have to know their pace and battle that pressure. If they are in the middle/rear, they will have the pressure to keep up and stay right on the tail of the group. If the roads are unknown to them, a more experienced rider should lead IMO. And no matter what, it should be made clear that they should ride their own pace. They should know that any time the group is turning from a route/road, we will hold at the intersection and group up....but separating on the route because of differences in pace is not only acceptable, it is more safe than struggling and riding above their comfort level.