You'll find the command on the right-click context menu for the Select Tool.
See https://help.sketchup.com/en/sketchup/o ... -whats-new
Invert Selection. Just like the title suggests, it's the small things that help your workflow! This new feature will allow you to select anything, then invert the selection of objects. This makes it simple to select items and then perform actions on their inverse. The keyboard shortcut for this will be: CTRL + SHIFT + I (Windows) or CMD + SHIFT + I (Mac).