This achievement is unlocked during your career, you need to complete between 5 and 7 events my final rally was Spain...
Once you complete your final rally, you will go back to your teams garage and have the option to join a new team or stay in the one your in...
If you scroll through the teams the Top left tells you what group that car is from, you can choose any WRC team, and as soon as the game starts loading with the WRC team your achievement should unlock