How do you give people elbow grease on car town?

How do you give people elbow grease on car town?
The car that the person is building will be in their garage in order to give them elbow grease you have to click on the car you wish to give elbow grease. The only exception to this rule is Police Charger that you have to complete in order to finish the Dom's Charger Challenge. I still have not figured out how to give someone elbow grease if it is in the other person's hideout.