In a Big World, which this one appears to be on at least three counts (spatially infinite open universe, inflationary scenario in Standard Model, and Everett branches), everyone who could exist already exists with probability 1. Thus, the issue is not so much creating new people, but ensuring that good things happen to people given that they exist. Creating a new person helps when you can provide them with good outcomes, because what you're really doing is increasing the frequency of good outcomes from that starting point.

Or at least that's one anthropic interpretation of ethics. But it is one reason why I don't endorse running out and creating lots of people if that lowers the average standard of living. In a Big World, it's the average standard of living that you care about.