Besides american express which other principal card company will agree to you be an authorised used and benefit from it im trying to boost my credit score and worthiness Answers:

im trying to boost my credit score and worthiness

Answers:
As of September 1st, piggybacking will no longer be valid.

This used to be a adjectives practice. Referred to as 'piggybacking', it was a loophole that allowed associates to become an 'authorized user' to take pre-eminence of someone else's credit history. But the loophole will be closed in September. Here's an article that discusses the change in more detail:

http://www.asapcreditcard.com/blog/credi...

Hope this help. GOOD LUCK!