In SketchUp only a Group or Component can be a solid. If you checked Entity Info after creating a Group from your cube, it should already have been designated as a solid. If it wasn’t, that suggests maybe you didn’t select all of the geometry of the cube (triple-click on a face) before making the Group.
Outer shell requires two solids to create the shell. You select the first solid either before or after activating the tool then select a second solid you want included within the shell (the cursor will be a little 1 if you need to select the first solid and a little 2 if you need to select the second). After the first operation, the result remains selected so you can continue adding more.
Got it … I didn’t understand “Outer Shell” … I thought that it would take a solid and create a new solid inside at some offset distance, i.e. creating a “shell”.
I found a plug-in that does this … so all is good.
Yeah, what outer shell does is to take two existing solids and create a new one that “wraps” their combined outer surfaces and eliminates any interior geometry if they overlap. Glad you found something that works for what you wanted.