I had a glance through the files I believe would be required, and this is sadly beyond my ability.
From what I can tell, it looks like you'd have to add a new label for each new group, which would require setting up a function that allows the player to create new groups. That would only allow the creation of groups, not allow the player to put slaves in the new group, which would be handled by a separate function.
I spent about 20 minutes digging through code, then another 30 minutes looking for examples online and eventually accepted that this is simply beyond my current skills.
Thanks for the suggestion, and I hope someone implements it because I really love the idea - especially since my latest save has over 70 slaves... (I am still making other mods and messing around too, so slave organization would be incredibly helpful)
If any other modders out there want to give this a go, please do so. I'd love for this feature to be implemented as well!