In Far Cry 5 you can invite a friend to play the story in co-op or join you on an expedition.
To invite someone to your co-op session, go to the title screen 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. You can pick someone from your friends list or use the search function to invite a specific online ID.
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 click on “Invite Friends”.
Alternatively, you can select “Quit”. This puts you back to the title screen from where you can invite.
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 “Host 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. It won’t save any story progress for the joining player.