Sweep riders. They box the car, so that the driver becomes aware of the other bikes. Sometimes they'll ride ahead of main group and look for potential trouble. They'll block so everyone can change lanes.
I guess you could call it aggressive sweep riding. It probably won't win any courtesy points from cagers! But then again, it keeps the group safe.
Link to original page on YouTube.
Not the most organized example, but you can see a couple of guys out of formation on the far left.