How many people need to ride along with the two bikes?

I can fit two bikes in my Prius easily. That is with the rear seats down, so there is only room for me and 1 passenger, but 2 people + 2 bikes works for me. There is also enough room for related stuff like a medium size cooler and a couple of duffle bags/backpacks.

I take the front tires off because it's easier to get the bikes in and out that way, but you could probably leave them on if you wanted.

The Prius gets me around fine in the snow conditions we get in the DC metro area, and that's with the stock tires on it. I have heard the Prius is also fine in snowier places if you put snow tires on it. A search of priuschat.com should confirm that.