Tom: Although the/Ø people who we don't know from our Sendai factory came to the party, the/Ø ones who we know didn't.
Sam: Oh, I didn't even know you have a factory in Sendai.
I could/would accept the use of "the" in both clauses, but definitely in the second because the persons are a specific/known/identifiable group of people. However, in the first reference, I could also accept no article because it is a general reference to an unknown and non-identifiable group.
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