To participate in the Online Co-op Campaign in Forza Horizon 3 and enjoy the experience with friends, follow these structured steps:

1. Unlock Online Play
To access online multiplayer features, you first need to make significant progress in the campaign. Here’s how:
– Progress through the Campaign: Focus on gaining fans and completing events which will help you unlock new festival areas.
– Build or Upgrade Festival Sites: Specifically, you will need to enhance a second festival site—like Surfers Paradise or the Outback—to at least level 2. This upgrade is essential, as it activates the online features, including co-op gameplay.

2. Form a Group
Once you’ve unlocked online play, you can group up with your friends:
– Use the Social Menu: Go to the in-game social menu to invite your friends into your group.
– Group Limitations: Keep in mind that the game supports a maximum of four players for co-operative play.

3. Start Co-op Campaign
With your group formed, it’s time to dive into the co-op campaign:
– Select Adventure Mode: The group leader should initiate the campaign by selecting Adventure mode.
– Event Participation: All members of the group will join in to participate in the events selected by the group leader.

4. Gameplay Features
Engaging in co-op mode offers various features that enhance your gaming experience:
– Team Races: All races in co-op are structured as Team Races against AI Drivatars. This competitive format encourages teamwork and strategy.
– Car Sharing: Players have the option to share cars within their team, enabling a broader range of gameplay styles and decisions.
– Event Availability: You and your friends can access all events available in the solo campaign, allowing for seamless progress through the game’s narrative together.

Additional Notes
– No Local Co-op: It’s important to note that Forza Horizon 3 does not support split-screen or local co-op gameplay; the multiplayer experience is exclusively online.

Summary
In conclusion, to enjoy the Online Co-op Campaign in Forza Horizon 3 with your friends, focus on upgrading festival sites to unlock online capabilities, form your group via the social menu, and allow the group leader to select the events. This approach ensures a vibrant and engaging cooperative experience as you progress through the campaign together.