I found out that you need qualify for the Horizon roster before you can unlock this achievement.
Once unlocked, I completed the showcase event and was able to play online.
Go to the Horizon Festival Drag Strip, located on the map. Press start, on the home tab click on Play Online - connect to Horizon Life. Once your in, invite your friend/s into the lobby. Press X to setup the event and scroll across to PVP.
Do this 5 times for the achievement.
If anything is slightly wrong with this guide, rather than voting down, leave a comment below as to how I can improve this guide. Thanks.