This should be easy to get but the setup is a little convoluted and method may not be obvious. I've seen messages from people struggling with it, so I will try to cover it in as much detail as possible.
The first step is to start a "Horizon Group". To do this, bring up the menu and from the Home tab, select "Horizon Group" (top row, right side). Press the Y key to bring up options and Invite Friends. Select the people you want to invite from your friends list and then go to the top of the list to Send Invite. Your friend(s) will receive the invite and have the opportunity to join your Horizon Group by pressing X.
Step 2 is to start a co-op session. With at least yourself and one other person in your Horizon Group, Bring up the menu again and this time go to the Social tab. Select Co-op Campaign (Bottom row, left side). It is up to you whether you want it to be a private or public campaign. Private will limit it to your own Horizon group. Otherwise, other people from your friend list can join (and in fact, may see a menu item offering the option to join your group).
Step 3 is for the leader (you, if you are the one taking all these steps) to start an event. This is a little counter-intuitive if you have been playing other Horizon games. You might expect to see a series of online event options to select once you are connected in a co-op campaign. This is where many become confused, because only the normal items on the map appear, so you may think your co-op session is broken. That is probably not the case. Simply choose any of the regular race events, including the blueprints created by your friends or club.
Once you select the event, other people in your Horizon Group will be offered the chance to join that even by pressing X. Simply complete the event and you will get a co-op event credit and this achievement. There is no need for anyone in your co-op team to win the event in order to get the achievements (i.e. random drivatar may win it).
If you are just trying to boost your way through this as quickly as possible, I suggest you make your co-op event the Coober Pedy Scramble, available in The Outback (I believe this one is available at a level 1 Outback Festival). Just blueprint a race before you make your co-op run, making it for 1 lap only. Each lap takes less than a minute, so if you blueprint such a race and do it 10 times in a row with your co-op team, you can knock out the entire co-op achievement sequence in about 10 or 15 minutes.
If someone in your Horizon Group gets knocked out of the race (this may happen after 15 seconds if your attention wavers), they simply need to go back to the Horizon Group menu option, press the Y button and then Join the group game again. If need be, the group leader (presumably you) can select the Horizon Group, press Y and re invite or invite additional friends.