I have read through the Documentation, Knowledgebase and watched all the videos but nowhere have I found an explanation of how Groups work. I believe that a user needs a simple mental model first and foremost. Otherwise you can imagine/fear complexity that just isn’t there. So this is the way I see it via simple-minded trial and error experiments. Hopefully clear and concise.
Groups are used to organize sets of Sketches and Shapes (also referred to as Bodies in the Documentation).
Sketches are automatically organized into a group based upon the plane or face (of a Shape) where they are located. A Sketch group can be Highlighted, Renamed or Deleted.
Shapes are automatically first organized into an automatically created group named Group of Shapes. All shapes you create automatically go here. If you create a New Group it is automatically highlighted and any new shapes created will go here. New shapes will always go to the currently highlighted group. A Shapes group can be Highlighted, Renamed or Deleted. Note that any particular Shape can only be a member of one Group at any given time. For example, if you have a Group of Red Shapes and another Group of Blue Shapes (say 3 shapes in each group) and you create a third Group named Red & Blue and add your red and blue shapes to this group, it will have 6 shapes while the Red and Blue Groups will now have 0 shapes. It is useful to perform simple Group operations while viewing the Groups display for a better understanding.
I hope the above is correct and that it can provide a mental model for a user intro to Groups.
