Just when Rockstar started cracking down on these, someone made a Co-op mod for GTA V.
Something many gamers have been missing from GTA Online is a proper co-operative game mode. The closest thing players have gotten is a set of team-based modes. However, there has been no PvE multiplayer option for players to share with their friends. In addition, the other GTA V multiplayer alternatives out there offering players some more freedom, such as modded servers and custom (not "content creator custom" but real custom) game modes and tweaks are being shut down one by one after Rockstar (check out the links for the reasons why).
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The Multiplayer Co-op mod gives players the ability to host private servers to play with their friends, against AI enemies. The mod takes a fairly simple approach to the affair. It does not add new missions or game modes, it just synchronizes the single player instances of each player. The NPCs move, attack, and die the same way in all of the connected player's instances. Also, an NPC is spawned in the game of each player representing the other players, who have the character model of whichever protagonist they're using, and the movements are synchronized as well. Pretty ingenious really.
This method also allows players to use mods in co-op as well, another thing people have been requesting to be made available in GTA Online in private lobbies (never going to happen).
Since this is a fan-made mod we are talking about, there are a few bugs to squash. Occasionally, the synchronization isn't very... synchronized. Player may be randomly booted from the server, though this is rare, and server hosting can still be a bit spiffy.
Is a cooperative PvE mode something you'd like to see in GTA Online?