How to make a new sensor for a group

[fraserb2]

I would like to split the lights currently on entity into a separate sensor for each floor of the house. I have added new groups in the device counter but I do not know how to make the final sensor for example (sensor.upstairs_lights_on, sensor.main_lights_on, etc). Is this done in the main home assistant setup or is there something in your setup that can facilitate adding these sensors