When assigned to one of your cars, crew members improve its abilities by reducing cooldown, boosting efficiency or extending their duration. Followers can only be used on the cars with abilities they boost.
Tap the "Crew" button on the main screen to manage your crew members. All available crew members are listed in the window. You can sort them by car they can be assigned to.
To assign a crew member to a car, go to Garage and choose one of the cars. If you have crew members that can be assigned to this car, you'll see a green "+" icon in these slots:
You can get new crew members as a reward for battle or by opening chests.