I did the MBR last weekend with a large group of mixed riders for a mate's stag do. It was fine. We just made sure we kept people aware of what was coming up and things to watch out for. Keep checking people have unlocked their forks if they lock them on the climbs and that people's QR's are tight.
Most of them struggled over the rocks at the start of the MBR, straight from the car park so we dropped down onto the blue (minotaur) and did the first section of that to the end of the berms and then went up the road climb from there. That might be a more welcome introduction?
Other bits to watch out for are the toothy section after the loose stone climb (sorry can't remember what it's called) and the section that brings you out on the open hillside, descending to one of the bridges; there are numerous large rock steps which are roll-able but do look quite intimidating. And you can approach them at speed, so someone launching themselves off them would probably end fairly badly.
I think the Tarw is a bit less technical.
If the group don't have cycling fitness, I imagine the Beast would be a real effort.