Group defense is still allowed for under the Non-Aggression Principle. If you nail the SOB who just killed your friend/relative/whatever, that isn't initiation either.
Theorists in this area commonly envision private security companies who are paid a small monthly fee in part to investigate and make arrests when you are the victim of a crime, regardless of the crime and post-mortem if necessary.
Another thing: not everybody who believes in the NAP is in favor of "zero government" (Anarcho-Capitalists). Both Libertarians and libertarians (not a repeat!) tend to believe that arbitrating and solving NAP violations is more or less the ONLY proper role for a limited government.
Libertarian thought says that modest property taxes and import duties would be enough to pay for courts and police, and a small core of a professional military to train and support the majority of the nation's defense forces, a citizen militia.