I've got a sublist on a suitelet and I would like to represent some lines as 'grouped' together, I was thinking perhaps do this by highlighting the lines but I can't seem to figure out how to do that with code, does anyone know? If there are alternative methods to do this I'd be interested to hear them too.

I am not pretty sure using suiteScript. have never tried it . But you can easily achieve it using Jquery or Javascript

I'm trying to avoid any DOM manipulation if possible but I may have to go down that route.

If you are not able to achieve using suiteScript best option is to go with DOM manipulation.