What will your hireling do? They will not passively gather resources for you but they will help you with your work. So for example if you will cut down trees and give the record of your work to your lumberjack, he will go out and gather the same resources for you. But only if you show him that you did your part as well. This way it's not possible to have hirelings and free resources everyday just for your short time in a game.
They could be also happy and sad. Don't forget to equip their house with a bed, table, trophies etc. If they will not be happy, they will not work.
And if they work hard of course they can level up. The bigger level they have, the more they cost but the higher level they can harvest.
The rules are simple and we will them soon in action. It could help newbies and veterans as well to get more stuff with less effort. And that is great.