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Well, seeing as Dark Eldar are pretty much completely lacking in morals, I imagine that they have a very unusual means of reproducing.
Firstly, there is the natural way. Dark Eldar are free to do pretty much anything they want to unless somebody stronger is directly opposed to it. In fact, all Eldar get so wrapped up and obsessed with things that I wouldn't be surprised if there are a few of them that have dedicated their lives to popping out as many babies as possible. Those little guys are probably entitled to a place in the Kabal that their mother is a part of.
The next method is completely conjecture, but it sounds like a Dark Eldar-y thing to do so I'm saying it anyway. I think that the various Homunculus clone themselves huge batches of young Eldar. They take a few for their experiments and art and turn the rest of them out onto the street where they are picked off by warp beasts and Hellions and Mandrakes. Only a handful survive until they reach adulthood, but those guys are bound to be badass.