Effect: One ally within 5 squares of you can use a free action to shift to a square adjacent to the target and then make a melee basic attack against it. If the attack hits, it deals 1d8 extra damage to the target, and the target has vulnerability to all damage equal to 1 + your Charisma modifier until the end of your next turn.
If I'm reading this right, then there is no maximum shift distance on this power, and no requirement as to which adjacent square the ally shifts to. So if my ally is a Rogue who is behind me at maximum range, then there is nothing to stop said Rogue from shifting all the way behind the target and into a flanking position with me? Thereby triggering CA for the attack, and Sneak Attack dice.