I invited someone to a board. They click the invite link and were allowed access to the board without requiring them to create an account. My board is only visible to members and is not public. They then forwarded the link to someone else (not a Trello user) and they too could access the board.
closed as off topic by ChrisF♦ Sep 26 '11 at 21:29
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The invitation link currently gives permission to view the board (so that people don't have to accept invitations without seeing that the board is legitimate). It sounds like the behavior you're expecting is that if you invite a current Trello user, then the invitation link should only be usable by that user when they are logged in. We'll consider making that change. Thanks!