There are various extensions that will slice a model in a variety of ways.
But it is such a fundamental of sketchup you need to get your head around it.
Here is a very simple shape, grouped. I position a face through the group, then enter the group for editing so that I am in the context of the groups faces. Triple click to select them all, then right click to choose Intersect faces with Model. This allows the external face to create edges within the faces inside the group. I select half of them and group them (the menu item offscreen) to easily separate them from the other half. Then trace an edge or two to infill the missing face. It takes far longer to type than to do.
