My car is sensitive to small rocks and bumps.?

I think you may have worn suspension parts on your car, I would go to an alignment shop and have everything checked out. 5 people in that car will slow it down, a lot. You have a small economy car with a small engine. It is normal for it to feel gutless and even more so when you add 600lbs worth of friends. If it hasn't had a tune up in a long time, that could help a little bit, but not much