Difficult to say much about this incident without having seen it, but here goes anyway!
Now, assuming that everyone involved is a reasonable sentient human-being, which is, after all the necessity for our highly developed civilisation to function, I have a couple of speculations to offer.
Given the above, it is unlikely that a ride leader would willingly lead a group of cyclists into the path of a fast moving car on a roundabout. Possible explanation? maybe the way was clear for the leader but not clear by the time the rest of the group were on the roundabout. In that case, I would suggest that following riders should do their own observations and stop if necessary. Maybe the situation on the roundabout changed very quickly. Possible in the circumstances I outline below.
Now, I have noticed an increasing tendency (and I have done this myself), particularly when a major road intersects a lesser one at the roundabout, for drivers to approach the roundabout with the speed they have carried from the main road. They (I) expect to gain instant priority on the roundabout as nothing much joins from the driver's right and they rarely need to stop. Maybe, since cyclists tend to prefer minor roads, this explains the event here. Never experienced it on a bike, but certainly in a car, I have been hooted at and flashed by a car which wasn't even on the roundabout when I pulled out on to it. My feelings tend to be, if you can hoot and flash then you can brake.
Maybe that is the key to the whole thing. Should vehicles be negotiating roundabouts at such a speed that unexpected events can only be dealt with by emergency stops? Possibly not.
So, recommendations.
Cyclists, look for yourself, don't blindly follow a leader.
Drivers, expect to brake if a vulnerable road user appears on a roundabout. In varying circumstances this could be cyclists, pram-pushers, pedestrians, dog-walkers. Remember who is operating the deadly machine, and is responsible for it. There is no such thing as right of way.