An "authorized user" is not contractually liable on a credit card account. You did not sign an application or a credit card agreement. Also, the debt may be barred by the Statute of Limitations (unless you send them a payment, which will re-start the SOL and may be a basis for them to argue that by paying on the account, you accepted responsibility.
You have two types of issues. First is their violation of the Fair Debt Collection Practices Act. Secondly, is the way that they are reporting the account to the credit reporting agencies. In order to take action on either of these, you should, first of all, consult an attorney.