All players can enjoy the multiplayer experience on Watch Dogs 2 if their in-game online settings allows it. The availability of some of the content is based upon your subscription status on your console.
If your game settings allows it, and you really catch the attention of the police in your game (i.e., get a 5-star ranking with the police), other players will be invited to track you down. Your objective is to evade them (though you can certainly try and go down with a fight).
You can also trigger this by selecting the Multiplayer option in your phone.
As a hunter, your role is to take down your bounty before it escapes. Bounty can appear spontaneously in your game, if your game settings allow it, or you can trigger the mode by going into the Multiplayer option on your phone.
Note that the role of hunter is only available to PS+ members.
PS4 players will need a PS+ membership to be able to invade other players.
Coop missions requires PS+ membership for both players.