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Unfortunately, this isn't possible, you can only add members to a card if they are members of the board.
If you add someone to the board, they will be able to see all cards on the board, it isn't possible to restrict the cards that they can see.
Sorry if this isn't the reply that you wanted!
Hello Community! My name is Brittany Joiner and I am a Trello enthusiast and Atlassian Community Leader. I'll be sharing with you a series of videos with my top tips on how to best use Trello...
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