A simple way is an IR/light sensor, aimed toward the point where a human's waist should be 3 to 6 feet ahead of the robot. Mount an IR/led on the human, transmitting a wavelength which the sensor is sensitive to (with filtering, etc., as needed).
A single photodetector probably is insufficient. Consider an array of 3 or 4 or 5. Aim each one slightly away from center. Steer the robot so they pick up equal intensities of light. Then it is steering directly for the human.
If you can achieve the above, then you might eventually do away with a sending light on the human. Just detect light/dark boundaries at the human's waist at the left and right sides.