Assassin's Creed Unity's story missions won't allow multiplayer
The big reveal for Ubisoft's Assassin's Creed Unity at E3 this past week is the ability to play with others cooperatively. As you probably saw in the gameplay video shown during Ubisoft's press conference, up to four players could play cooperatively in a mission that involved infiltrating a French palace and assassinating Marquis De Bullion.
However, during Ubisoft's behind-closed-doors preview, it was revealed that only certain missions will allow multiplayer gameplay. Much of the game's story can only be played single player.
So what can you play with others? Well, as it was explained to me, the entire open-world -- outside of story missions -- will allow for multiplayer. Friends can join in to help kill guards, find collectibles, and complete certain multiplayer-focused missions (which can also be completed alone as well).
The good news is that when playing with others, continuing with your story will simply transport you to a separate instance. When complete, I assume -- as this next part was never confirmed to me -- it'll transport you back into the open-world where you can continue exploring with your friends.