adding an authorized user

[Anonymous]

if the purpose were to attach a credit history, does it matter whether that AU made a charge on the credit card (i.e. would the AU still get the credit history if no charges were made on his AU card)? or does it vary across CRAs?

[llecs]

It varies across creditors. Some report AUs and some don't and use of that AU CC doesn't matter. The history still carrys over. In some cases the open date won't transfer, like in Amex.

[RobertEG]

If the creditor has reported the account to the user's credit file, it is not necessary for the AU to use the account.

In fact, the best way to ensure that an AU does not screw up the credit of the account owner is to not even provide them a card for their use.