40 or 50 pounds per square foot (psf) live-load is typical for residential construction. But, that's a MINIMUM, not the point at which the structure fails since that rating has a large safety margin already built in. What that means is that exceeding the 40 psf rating will result in more deflection, not a failure of the system. Failure would likely take several times the rated live-load.
Think of how much a waterbed weighs... With a couple people in a king-size waterbed with the frame and headboard, etc., you're probably putting almost 2000 pounds in less than 80 sqft - or around 25 psf. In fact, you could put EIGHT people in that same waterbed and not exceed the 40psf live-load on the floor. There's an image for you!
Bottom line is that unless you build your riser out of 1/4" plate steel or from a solid block of MDF, you'll be well within typical (and safe) limits - even with 6 people all bouncing in their chairs.