Harry Potter Question..... Sirius Black?

So I'm reading the 5th book and have all of a sudden gotten confused. I need refreshing on a few facts from book 3... So if Dumbledore found out about Sirius, Remus, James, and Peter helping Lupin during his werewolfness, why didn't he report them to the ministry as being unregistered animagus's? Or did he? So, is Sirius registered or unregistered? Also, as a side question.. What does is take to be a registered animagus and why would they need to be registered? Not many animagus's are registered anyways, right? Thanks! I have obviously forgotten a lot :/