Becoming an AU is very easy - some banks let the person just add an extra card over their websites (they would need your SSN). My mother did this for me (and I'm still on a few of them) when I wasn't old enough to get a card.
The warning, as already mentioned, is that the good and bad will be equal - that is, if they do anything wrong (like my mother charging up to 90% of the limit, or being late ONE time for a 30-day hit) then it will also reflect on you. However, that's easy enough to fix as well - just get removed from the card (Assuming you now have your own).
Also, quickly, there's no such thing as "negative credit" - you have a score or you dont - and that score will start in the 300s if it's really really bad. But you can't have a negative. Usually for an AU, the bank assumes the primary user will be taking charge and thus they won't pull you.
As per your signature, a "secure" card also helps - I would do both if you trust the person adding you. Good luck!
