A medium steak should have a thick band of light pink through the middle but, more browned than pink. The sides should be a rich brown color and the top and bottom charred darkly (but no black). This steak should have some play through the middle but feel firm to the touch.
Grill this steak over a medium high heat for about 5 to 6 minutes per side (depending on your grill).
If you are grilling for a large group of people this is the best level of doneness that pleases most everyone.