Google now is an extension of the search function, so you could use the search voice function to issue a navigate command. However due to my accent I find that it causes issues.
However you can navigate directly to a person via the contact app. when the contact app is open, search for contact you want (you need to be in the contact tab, not the phone tab - favourites tab will work too). Press on the person's picture (or icon if no picture) - this will pop up an action bar of everything you can do with the contact, from phoning, e-mailing, facebook etc, and if you have an address for that person, by scrolling the action bar you should see the map icon, which if pressed provides their address in the action box. Press the address, choose maps if it isn't already your always use. If you have navigation set up to be your standard on the bubble bar press the navigation option, otherwise press on the bubble bar to bring up the address, and choose the navigation option. If directions only option, use that and when directions shows details to get there, use the navigation button on bottom right to go into it.