This Achievement is unlocked and awarded for acquiring 500K XP for your Car Club. This may seem a bit more complicated on the face of it than it actually is...
The first point that needs making is this: You CAN unlock his Achievement alone. You don't actually need other members of a Car Club or join one - you can simply create one yourself to do this.
Just create a Car Club (or join one) as soon as you can - and "as soon as you can" ttranslates to immediately after completing the Train Showcase Challenge.
So either join a Club or make your own, and then simply continue to play - because pretty much all of the XP you earn will apply from then on...