I hope this explains it:

In the menu, under the current job, if you look where it says "Stage 5" and to the right of it, it tells how much money you make from the that stage. Now, if you look to the right of this, you should see two little icons: A hammer in a circle, and one that looks like a face wearing a construction helmet. If you hoover over the one with the face wearing the construction helmet, it should say: "Engage worker"

This will engage the worker in the selected stage of the construction.

Works the same way with the little menu in the opper right corner, just below where you can see your companys current money and your current level.

Hope this helps.

Thank you

alright, but can i help a worker that i have assigned?

i assigned a worker to do a massive job and it has taken 4 hours just to get 50%
can i help them finish the task or there's nothing i can do but wait?