Hi @TG ,
Welcome to the community!
Well an Observer of a board can copy the board to there own account!
If your board is a private board then you can add/invite users using these documentations.
https://help.trello.com/article/717-adding-people-to-a-board
https://help.trello.com/article/1024-invite-people-by-using-a-shareable-link
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