I've rescued thousands of NPR crew and unloaded them at a colony, I couldn't see anything happening with them so I just added pop to the colony for the survivors.
However I've noticed I have 6421 crew available which seems rather excessive, perhaps maybe they are being rescued?
I have an academy level of 2, which grants 10 officers a year, my training level is set at 5 and I'm on year 12. I appear to be getting 9 crew per 5 days, which would give me something like 7500 crew since the game began. This fits in well with what I have, but it would be nice if rescued friendly or allied crew joined your officers, rescued hostiles becoming POWs. Neutrals joining your population maybe? Or maybe a chance of some joining your crew, or population, or POW depending on relations.