Personally, as MOST models told me, they prefer one-to-one shoot so that they can know the photographer better.
There is absolutely no question that one-one is the most ideal, both from the photographer and model's point of view.
But how much are you willing to pay?
How much are other photographers willing to pay?
Because there is a (mainly) financial constraint, there exists group shoots. So as what the TS asked, how best to organise such shoots?