Different locations do it differently. I know of one are where all you need is someone with an ID to vouch that you are a resident of that area and you can register (under any name you want) and vote. Poll watchers have told me about cases where a bus shows up one person vouches for everybody on the bus and so they vote. They are sure it is fraud, but nothing can be done about it.

If it is true or not I don't know, but is a real concern if so.