Simply complete the event and you will get a co-op event credit and this achievement. Once you select the event, other people in your Horizon Group will be offered the chance to join that even by pressing X. Simply choose any of the regular race events, including the blueprints created by your friends or club. 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.
You might expect to see a series of online event options to select once you are connected in a co-op campaign. This is a little counter-intuitive if you have been playing other Horizon games.
Step 3 is for the leader (you, if you are the one taking all these steps) to start an event. Otherwise, other people from your friend list can join (and in fact, may see a menu item offering the option to join your group). Private will limit it to your own Horizon group. It is up to you whether you want it to be a private or public campaign. Select Co-op Campaign (Bottom row, left side). 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. Your friend(s) will receive the invite and have the opportunity to join your Horizon Group by pressing X. Select the people you want to invite from your friends list and then go to the top of the list to Send Invite. Press the Y key to bring up options and Invite Friends. To do this, bring up the menu and from the Home tab, select "Horizon Group" (top row, right side). The first step is to start a "Horizon Group". I've seen messages from people struggling with it, so I will try to cover it in as much detail as possible. This should be easy to get but the setup is a little convoluted and method may not be obvious.