Okay, I figured it out. I needed to draw another rectangle on top of the slab, then offset it, then delete the inside face to avoid having two faces.

Basically, I drew the rectangular slab, pulled it, made it into a group, I clicked outside to exit, now I want to offset the slab to make exterior walls.

I'm trying to render a 2 story building. I have grouped my first floor foundation, but when I try to continue to build on top of it, SketchUp only allows me to edit the group. ??