Looking for some advice:

I have added someone as an authorized user on one of my cards. I am not allowing them to have their own card but am monitoring their monthly payments (I used my own card to make the purchase). I have considered adding them as an authorized user to may two other accounts that have been open for several years and have a decent credit limit. I will do the same thing, which is not allow them to have their own card or even make purchases on the account, but I have considered doing this solely to help their credit score. 

Would this be a good idea or a bad idea? Also, what could happen if or when they are removed as an authorized user?

Thank you