Watch Dogs will have an interesting multiplayer options that will allow other online players to join uninvited to an open sessions, from which they can choose to do a variety of things.
The full multiplayer experience has not yet been detailed, but it's understood that the system will allow other players to drop in and out as they please, and this opens up open lobbied players to a lot of potential griefing.
Whilst the option is there for players to work together, Ubisoft has taken into consideration that some bad eggs may attempt to join open lobbies simply to ruin the experience of others, and according to lead game designer Danny Belanger, there will be a bunch of integrated features to allow lobby hosts to combat griefing.
Firstly, if players simply want to play on their own, they can completely switch off the drop in, drop out multiplayer, which will completely cut any ties with any other players, and allow people to play through the game by themselves as a single player experience.
Danny Belanger also says there will be other mechanics in the game to make it harder for players to ruin the fun of others. A system called the shield will be integrated that will stop players from repeatedly connecting to another person's game, unless that particular person is also aggressively joining other's lobbies.
So far, no more details have been given on the shield or any of the other multiplayer options, but I expect more will be revealed by Ubisoft shortly.