You need to have four legendary crew members equipped simultaneously - they must be in your active crew, reserves do not count.
In the bottom right corner of each crew card is a colour:
grey - common
green - frequent
blue - rare
purple - very rare
orange - legendary
You need to have 4 cards equipped at the same time that are orange.
You can win crew cards after each rally, and also some become available to purchase from the 'recruitment' option of the crew management section.
Use the Team upgrade path in R&D to maximise chances of getting legendary crew, the perk you want is conveniently called 'Legendary'. At most, it will cost you 5 upgrade points (earned from levelling up) to acquire this perk.
As soon as you equip all 4, the achievement unlocks, you don't need to complete a race with them.
I unlocked this right at the end of my WRC2 Pro campaign.