Landlords charging me for previous tenants' damage - HELP!?

You absolutely don't have to pay for it- as long as you noted it, took pictures or videos before moving in, and notified the landlord to prove that it was pre-existing damage. I'm not sure what groups, specifically, could help with this, but I bet that if you call the police department or Better Business Bureau, they can direct you to the appropriate place. I would suggest getting out of that place as soon as possible- they're doing their best to break the law with you. You could definitely win a case in small claims court if you already paid and have evidence that the damage was pre-existing (said photos or videos with dates, etc.).