Most of your guests will have their phones out anyway to film your big day, but just in case, here are some tips:
You know your uncle who films absolutely everything at every family event? Ask him!
Your friend who instagrams her entire life? Ask her!
Most of your guests will tend to open up in front of people whom they know best. So, your side of the family will be most candid if someone from your side films them; same thing with your friends.