It depends on the size of the group.

If the group is just a handful of riders, single file is practical, as the passing vehicle doesn't have to go far while passing.

But if the group is large, single file is unpractical, as it would require passing vehicles to go a long distance to pass.

On our Saturday group ride, it's common to have 50 or more riders. When there's traffic behind, we double up and move to the right half of the lane.