Hi, welcome.
How are the odometer and miles since last fuel up calculated? Is there an inherent difference in the end result of MPGs?
No, there's no difference in the long run. Take a look at this FAQ:
What's the difference between tracking with Trip Miles vs. Odometer?
Is there a way to add multiple drivers for a car to compare driving habits?
Yes, you can share a car with another driver so that they can add fuel-ups as well. But there's no feature that compares the two different drivers in any way. One thing you could do is add a note to each fuel-up indicating who added it, and you use that to help you remember who did what, when. But the only information Fuelly is tracking is fuel-ups. There's no indication who was driving during a particular time period so I'm not sure a feature looking at two different people who added fuel to a vehicle would tell you much.