Most NPCs have a fairly diverse starting skill set so raising skills from nothing isn’t really an issue. It would be more important to focus on finding that one or two who are good at what you need them to be.
So, I’ll look at a basic implementation as:
just you, 100% time 100% Exp
you + 1, 75% time, 75% Exp you, 75% Exp npc
you + 2, 60% time, 50% Exp you, 50% exp to each npc
The bonus only kicks in if you are doing batch crafting and the time will never be less than that of a single item.
I might throw is a 10% exp kick to all npcs who are watching you craft, regardless of whether or not they can help… that would let you ‘teach’ your followers from the ground up.
Sound like a decent starting point?
EDIT:
tested
alone -> 5 clean water in 10min
1 npc -> 8
2 npc -> 11
on to exp!