I vote for open registration. IMO, closing or limiting registration is going against the very ideas of freedom totse was founded on. Even if we make it invite only, the CP spammers who want to get in will manipulate and trick other users into getting codes.
This debate over the registration system feels very similar to the new war on gun ownership rights in the US. Everyone's afraid of a few crazies so they're trying to change the whole system because of it.