Back in the day, you only need one copy of a game to play multiplayer with people. Warcraft 2 and StarCraft come to mind. Throughout the years, there were some that still held on to that feature.
Sony has announced that on the PlayStation 3 and PlayStation 4, you'll be able to play co-op Far Cry 4 with just one copy of the game. That. Is. Awesome.
Far Cry 4 looks to be a pretty solid entry to the series and having the ability to play co-op, let alone co-op with one copy of the game, is just icing on the cake.