New account registrations are currently either closed or very limited
If you do that, the psychology goes like this: "I wonder what this is?" (click) "Ooh, a new social community! I like those!" (reads further) "What do you mean, new accounts are closed or very limited? What if I want to join? Why are you gatekeeping me? Damn you!"... except with words stronger than 'damn'.
Once you're ready to let anyone sign up, email hn@ycombinator.com and we'll help you out.