1) you will need to register general POA in concerned sub-registrar office. But in this circumstances NOC will be better.
2) NOC do not need registration. Your friends needs to mention in NOC that they have received their share of amount from you and have no objection in flat getting duly registered on your sole and whole name. They relinquish all their rights in the property with the NOC. and then the signature of you all along with witness must be there.