Far Cry 4

Having a friend is not necessary, you can join the game of any person on the planet and capture any outpost or fortress with him, provided that he plays the campaign online and not offline

For October 2022
It is better to look for a partner through the main menu of the game - join a game session campaign - if you offer help from the game, the connection with the server is lost

In the main menu, select joining a joint game. The search for available players begins, which can be quite long. Finally, he drops it into a session with a player who is playing online. We immediately turn on the map and check whether this player has any unliberated outposts. If there is, we put a mark on the map, we go there, we free it - the trophy is yours.

I saw it a lot on the forums and I myself encountered a problem playing in co-op
We couldn't join each other - it said "Error, try again later"
Moreover, if we made an open game, then random people connected to us in seconds. And if they themselves tried to find a random game (not with each other), then the search hung endlessly.

I tried all the network tricks that Yubiki recommends doing (open the necessary ports, connect via wire, etc.) - nothing helped.
In the end, the solution turned out to be banal - my friend went further along the plot (opened half a map somewhere), and then he was able to follow my invitation, and also calmly invite me (at the same time, I still can’t find anyone by searching because I haven’t yet didn't go far enough). So, apparently, a “full-fledged” cooperative opens up much further than clearing the first outpost, since being at the stage of the plot with only 1 captured outpost we were unable to connect anywhere. So I recommend taking on co-op trophies after clearing at least half the map, and if even then it doesn’t work out, then start panicking with the network settings
Just in case, this guide will help someone
But I repeat - first try to connect after walking halfway through the map and start setting up the network only if you still can’t connect

PS Today we tried it with another person who couldn’t connect to anyone and couldn’t reach him - going through the story until he got a house helped and he was able to connect to a friend and accept the invitation himself. So if you have an error - 1 of you must follow the storyline to get the house

I’ll correct the hint from PeoplePower - to get the trophy, you first need to unlock the character Hurk in a single company. According to the plot, we clear the first outpost. Further up the hill behind the outpost there is a homemade aircraft (local helicopter) standing near the boxes. and the letter N icon is on. We approach and a pickup truck with Hurk in the back pulls up and he makes a speech about support and everything and gives a fist bump. You bumped fists and he left. After this, a notification appears in the game that the game has become available in online mode for completion and you can play as Hurk.
Accordingly, your friend should open the same thing in the game. And after that, create a game and invite your friend to a closed session. Voila and you immediately take the trophy with the killer of the driver of the vehicle from the passenger seat. Then you restart the session - your friend creates the game and invites you. Now you are Hurk - you intercept any outpost/fortress and the trophy is yours.