The payload rating of the Prius is 800 pounds. That is, once you have the car filled up with fluids you can put 800 pounds of people plus cargo inside and everything will be fine. Of course, this does include the driver.
The car won't collapse in a heap if you exceed this payload, but you would be well advised to be cautious. For example, avoid steep hills and very high speeds. This is the same with any car.
Toyota do not recommend towing with the Prius. If we apply a bit of sense, we can see that towing a small trailer would be OK if we take it steady. If you need to exceed 800 pounds by a significant margin, you might want to go the small trailer route to get some of the payload off the car's suspension.
The figure of 300 pounds might come from the Honda Insight, which is a much smaller car. Currently, the payload rating for the Insight is 365 pounds, except on leap years, when it is 366.