'Sup Savings 321,
The answer to your question in most cases is yes, if you want to be added as a joint account holder an inquiry will be made to determine your creditworthiness. My wife and I had a few CCs that we'd individually acquired before we were married. When we tried to add each other to the cards as joint account holders we were consistently told that the person being added would have to apply and be approved, but if one of us added the other person as an authorized user, no application was necessary. We assumed this is because the original account holder was still legally responsible for the debt. In one instance, my wife and I had CCs with the same company, with strong payment histories, and that company merged our accounts into a single account as a courtesy without doing an inquiry. With regard to your score, I'd just go for the AU and not apply for the joint, because the hard inquiries will definitely hurt your score and you might not get approved for any of the CCs. You should also note that it may take some time (up to 2 or 3 months) for the CRA's to update your file and for the updates to reflect in your score. In Sept AU accounts will not be used in the scoring process so you might have already missed the window of opportunity to benefit from having an AU in your file. (It's already August so you only have a month before the new rules, if the CRA's take a while to update your file, you won't benefit from your new AU info)
Good Luck!