Invading a friend while he is playing

I would love to do a hack or trail mission against one of my friends.

How can I invade a friends game?

Solution 1:

According to Ubisoft Support's Twitter account the invasion mode is random only.

@UbisoftSupport is there anyway you can hack your friends in watch dogs? I've been trying to figure out how to join friends games to hack

@MyPizzaIsWarm No this multiplayer mode is random. [S]

Solution 2:

You can with the mobile app! From the mobile app FAQ here: Can I Play With Anyone I Want?

Yep, you sure can. To start off, you’ll need to log in with your Xbox Live, PlayStation Network or Uplay account. If you don’t have an account with any of the above, you can easily create a new one, using a free PSN or Xbox Live Silver account. (Uplay is always free.) Then, you simply enter your friend’s ID, and he’ll pop up in your system; if he’s available, you can invite him to play with you. Remember, though, you need to be logged in to an account on the same system as your friend in order to play with him.

The mobile game is pretty fun! I haven't played it myself, but I've had a friend trying to kill me while sitting on the couch next to me and we both had a blast!