Note: I have this Xbox achievement and have helped many people to unlock it there.
This achievement is easy. You need to be Ubisoft app friends with the person; in Crew Management, you can choose to add a Crew member and pick a friend who is online, obviously.
Then, you need to travel 50 km in a vehicle. Unlike the description says, you don't need to "drive". The fastest way to unlock this is flying a fast plane for 50 km. Where you fly doesn't matter; straight line, full speed is best.
You don't have to be close to each other, or even see each other at any time, just don't start race events and remain in freeroam with your friend in the Crew list!