Convoys will unlock when you get to the second festival site (so Outback or Surfers depending on what you choose).
To add Drivatars to your convoy, press the right stick when they are near you to beep at them. You will see the UI above them turn orange when they are added to your convoy.
To get this achievement you need to drive in a convoy for a TOTAL of 100 miles - this is not all in one go.
As long as you beep at a Drivatar when driving between events and stunt you will get this as you progress through the game. When you enter an event your convoy will be disbanded so be sure to beep at a Drivatar after every event to get in a new convoy.
If you want to grind this out a little quicker then the best thing is to drive up and down the freeway in a fast car. This is not necessary though if you follow the instructions above.