WhereYouAt points the way towards your group members. Whenever a group member is selected a hint arrow will point towards them.
You can target your groupmember by clicking on their name (unit frame), or by pressing one of the group selection hotkeys (by default, these keys are F2, F3, F4, and F5)
If your target is close to you, i.e. they're in the same general area as you, then a hint arrow will appear. If they're within 5 meters or so, lightning bolts will appear over their head. Nothing too fancy, just the sort of hint arrows that you get for quest objectives and the like.