adding an AU does not impact your credit score one bit. What does affect your credit score is if the AU you added run up the balance on the card and only pays the minimum monthly. having high balance on your card reporting to the bureaus can affect your score.
Does adding an authorized user with poor credit to your account impact ones own credit rating at all?
Think of it this way: The effect is that the AU gets your TL reported on his/her reports. The AU isn't reported on your reports.
That said, the AU can certainly affect your utilization and possibly payment history if the AU racks up charges you can't cover. If this is a concern you don't have to provide the card to the AU if you only want the TL to report for the AU.
Also keep in mind that not all creditors report for AU's. My wife is an AU on my PenFed card and it doesn't appear on her reports.