You get this by going to your phone in game, then to the multiplayer option, then to Hacking Invasion. After this, you will be placed in someone's game and you must not be identified while remaining in a circle to hack them.
I recommend standing up and circling around cars to avoid being scanned. The way I got it was by hovering around a van while the other player searched for me. Basically, just try to find the best hiding place in the circle. You can choose when to hack them, so might be smart to wait until you know a spot to hide.
EDIT: To make this really easy, invade a friend. I let a friend fully hack me with the option in the multiplayer tab and he got the achievement.