How do I add someone to a Facebook privacy group that I have previously created?

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There are certain people I want to add to a certain privacy group that I have created on my Facebook account because I want these people to see only my limited account. How do I add them to a privacy group that already exists? I've been trying to figure it out for weeks now, but can't seem to see the link or buttons to click on to do this. Or is this even possible at all?