This is something I've just recently started thinking about and why not earlier, I don't know. Anyway I don't always use my bike (Akuma) and it's even less common for me to have someone riding along with me. Now it seems like the weight of the extra person does indeed play a factor in limiting the bike's acceleration and perhaps top speed. It also seems to have an effect on the handling (as it also should due to an extra ass on the bike).
I'm wondering just specifically how much it limits it (for example - 30% reduction?), even if we don't know an exact amount it does feel significant. It never really crossed my mind because I also don't really ride with more than 1 other person in a car, but do you feel a 2nd person in a 2 seater has any effect? How about 4 people in a 4 person vehicle? Or is this really only just with bikes?