On rural roads, I ride the fog line, but move over when traffic comes up behind me.
The primary reason is to minimize exposure to glass, gravel, and debris.
For group rides, it would depend on the road, traffic, conditions, and group. Side-by-side?
Still, if it is truly a full unidirectional traffic lane path that is clean, there is no reason not to be several feet into the middle of the lane.
Approaching critical intersections that could have right hooks?