If you are the primary cardholder, their behavior can affect you is if they run that particular card up over the limit or something else dumb, leaving you stuck with the balance. There have been some posts here about AU's changing the mail address and e-mail for the contact, so the actual cardholder doesn't know what's going on. (Which is why you don't rely solely on communication from the CCC.)
If you are the AU, the only way you are affected is by the history and the future behavior on that one card. If there are problems on it, or if the cardholder stars getting behind, this will show on your report, and you will need to get yourself removed from the card pronto.
The card is the link between the two otherwise unrelated histories.
* Credit is a wonderful servant, but a terrible master. * Who's the boss --you or your credit?
FICO's: EQ 781 - TU 793 - EX 779 (from PSECU) - Done credit hunting; having fun with credit gardening. - EQ 590 on 5/14/2007