One automation per space (set of lights) you would like to control.
The group you talking about is if you have 2 sensors that control a space (set of lights) then it best to group them. Say you have a pantry / closet that has 2 + doors with 2 + contact sensors then you would group them.
For the motion group here is an example. Say you have kitchen lights controlled by an automation using motion sensors to turn the light ON and OFF like my 
Sensor Light blueprint. You then have your kitchen pantry with door contact sensor that will turn ON your pantry light when you open the door. This automation will turn the light ON and OFF in a set amount of time but if you use the motion sensor option this will keep your pantry light ON when motion is detected (motion sensor for your kitchen lights). In your kitchen you may have 2 motion sensors to control the kitchen lights, if you do then it is best to group them and use the group. If you walk out of the kitchen and leave the pantry door open all you lights will turn OFF (kitchen & pantry lights). When you walk back into the kitchen then all your lights will turn ON again.
Hope this helps you understand this automation.
Blacky 