Users can become a driver of another vehicle by sending a request to that vehicle owner. Vehicle owners also can send invites to other users to become a driver for their vehicle.  If the user wishes to no longer be the driver, he has the option to opt out. Both the scenarios are illustrated below:

Scenario1: Vehicle Owner of  a test vehicle here called FG6785 is inviting a test user named "both 130" to become a driver. 

The 2nd user now receives the notification both via email and on the App. He can accept /deny the request.:

The requester now sees that the user "both 130" has now become a driver for his vehicle: Of course he gets a notification too about it.

Scenario 2:

When a user wants to become a driver for a particular vehicle, he can do so by entering the Vehicle license plate number on the Add vehicle screen. The user is prompted with the message "Do you want to become a driver for this vehicle" - on clicking yes, the owner is notified. and the Owner now has the option to accept or decline as shown:

Owner notifications and accept /decline options: Once accepted, the Owner can now see the driver name in his vehicle profile.

The user who originated the request can also see the vehicle in his vehicle profile, stating that he is a driver. Of course he receives the notifications as and when the owner accepts/declines.