In Far Cry 5 you can invite a friend to play the story in co-op. In arcade mode a total of six players can be invited to a party.
To invite someone to your co-op session, go to the main menu and press the Touchpad (PS4) / View-Button (Xbox One). It says “In-Game Party” in the bottom right corner. Click “Invite Player” and select someone from your friends list. There is no matchmaking for story co-op. It only works with two players, and only people on your friends list can be invited.
If you’re already in the game’s story mode you can press Touchpad (PS4) / View-Button (XB1) and scroll to the online section, then press “Invite Friends”.
Alternatively, you can select “Quit”. This puts you back to the main menu from where you can invite. You can also start arcade mode directly from the main menu after inviting your friends. In Arcade there is Multiplayer matchmaking. So if your team is not full you can search for additional players via matchmaking.
When joining someone for the story mode, be aware that story-related trophies & achievements can only be unlocked by the host, not by the joining player (everything that says Solo Campaign only in the description can be unlocked in co-op but only by the host). It will load the game world & mission progress of the host’s save. The progress of the joining player does not matter. Even if your friend is much further in the game you can join and play together.