Range: 100lb driver vs 200 lb driver vs 800lbs of people?

What effect does driver and passenger combined weight haveon range? How much better range will my 100lb wife get relative to me at 200lbs? What if I have (4) 200lb people in the car?

If you drive mostly on flat highways, occupant weight won't have much effect. If you drive mostly stop-and-go and/or drive over mostly hilly terrain and/or usually accelerate briskly, occupant weight would have a bigger effect on range. I.e., it's difficult to quantify the effect of occupant weight on range.

Remember high school physics? force=1/2*mass*velocity^2. As a percentage of the whole vehicle, it's not much difference. Is it measurable, yes, with the proper equipment, but theoretically, you could calculate it. In theory, more mass would help with regenerative braking, since there's more kinetic energy available, but, it takes more energy to achieve your cruising speed. Regen is not 100%, so there are always losses. As said, cruising on the level, almost no difference (depends on the changes in drag caused by the tires - offset maybe by raising the tire pressure; or the angle the vehicle sits, changing the Cd), it's only when trying to go up a hill or accelerating when it has any significant effect. In the winter, when heating is required, it might be MORE efficient, since body heat will offset some of the heating required!

It reminds of the last time I bought a mountain bike and for another $500 I could get one that was a little lighter. I decided it was much cheaper to lose 5 lbs.

Practically it doesn't sound like anyone is seeing much of a difference in range when carrying passengers. I'm sure there is one, but not noticable. As suggested I suppose the % decrease in range would be the added mass divided by the weight of the vehicle (3,000 lbs). Less than 3% for kids and less 7% for most adults. Does that sound about right?

On the other hand given how much effort BMW expended to minimize the weight of the vehicle, they must think it makes a big difference.

Decreasing the weight of the vehicle means always maximizing the range, even when you're not carrying around maximum load. But, it also means that the chassis CAN carry more and stay in its weight/size class. Most people don't always drive with the vehicle at full load, so saving those extra pounds helps more often.

You already have your answer to an extent in the performance / power usage differential of the i3 REX vs BEV.

Obviously there are other factors such as the drag and the difference in cabin heating to consider.