Parfit argued that this argument led to the "Repugnant Conclusion," the idea that the best sort of world is one with a large population with lives barely worth living.

So the Mere Addition Paradox doesn't prove what Parfit thought it did? We don't have any moral obligation to tile the universe with people whose lives are barely worth living?

I'm pretty sure that this is not what Parfit was arguing.

As I understand it, Parfit's Repugnant Conclusion was that, given any possible world (even one with billions of people who each have an extremely high quality of life), there is a better possible world in which everyone has a life that is barely worth living (better because the population is much larger, and "barely worth living" is better than nothing). The argument he made was that the Repugnant Conclusion followed from most theories of population ethics (that is, most attempts to define "better" in this context), but most people refused to accept it.

That does not mean that a high-population low-quality-of-life world is the best possible world; a possible world with the same high population and higher quality of life would be even better. And it does not necessarily mean that we should strive for a world with high population and low quality of life; which possible world we should strive for depends on which possible worlds are reachable from here. But it does mean accepting that a hypothetical possible World 1, where there are lots of people and everyone has a life that is barely worth living, is better than a hypothetical possible World 2, where there are fewer people (though perhaps still billions or more) and everyone has a high quality of life. Many people refuse to accept this conclusion and find it repugnant, even if it is implied by the moral theory that they endorse.