You can add up to four robots to the grid and control the movement of each robot one at a time. Click the "Add Robot" button to add a robot or the "Remove Robot" button to remove a robot. When you add a robot you will see a new block menu appear in the middle section labeled "Robot 2" (or "Robot 3" or "Robot 4"). Each robot is assigned a color so you know which blocks to use to control which robot. 

Watch the video to learn about adding and controlling multiple robots.