Gather a even numbered amount of users (4,6,8 as there needs to be balance for Team gameplay), head off to Pro Tour lobby and set up Teams of 2, 3 or 4 users respectively. Choices are solo and team so make sure you select "team" in the Pro Tour lobby.
With a vocal countdown hit "search matches" for Team Transporter and with some luck everyone should get linked up in the same lobby without randoms and the timer will start to countdown. Each user will take a turn capturing the flag first, next user steals it, rinse and repeat until each user has stolen this 1st flag once. Once this is completed get any one of the users to transport this flag to the purple base, in order for the 2nd flag to populate, capture this one then head to a central area in the compound free of obstructions.
Now congregate around this user with this "last" flag and take turns ramming each other, tracking amount of steals as you go along. Once you have stolen 5 flags the achievement will pop-up in-game (unlike the other D3 cheevos which pop-up after all the loading screens disappear after race results).
Best news is all the users can take their time in this mode as there is no timer for Transporter, just target amount of flags to be returned to base which you won't be doing anyways for this unlock.