There's no way to do it natively within Companion. In these example pics I did just make a set of lines on different sides, saved them out as different colours and placed them on the buttons purely to show similar button groupings. I only did it for the examples and I don't use them in actual production. If you need to differentiate different groups of buttons I'd recommend changing the default backgrounds of the buttons and having them slightly off-black, so they have just enough colour to differentiate them but it's still clear when the feedback is not active.