There are some aircrafts that can be used both for singlepilot-ops, and for multipilot-ops. This requires that the plane can legally be flown single pilot. An operater (or private person) can then choose to get the plane approved for multi pilots as well.
The advantage here is that the co-pilot does not need an MCC to fly right seat. And when he has flown x amount of hours (I have forgotten the amount) he will automatically earn an MCC.
We did that in my company (AOC) with a Golden Eagle. The advantage is that if you cannot find a co-pilot, it can be flown by just 1 pilot from left seat. And if there was a co-pilot able to fly, he would earn hours, and eventually get an MCC.
But as far as I remember any multicrew stuff requires CPL/ATPL. It cannot be done on a PPL.
So in short a PPL, can fly any single-pilot approved aircraft with as many safety pilots he wants. But if he wants multicrew he needs a CPL...(and a lot of hours to fly left seat)