When I start typing to add my friends in Trello, I see all these people I don't know! What's up with that? Are you showing my boards to the public?
We don't let you see other people's boards unless they've been explicitly shared with you or made public.
We do let anyone invite you, or you invite anyone, to a board, in the same way you can invite anyone to link to you on a social network like Facebook or LinkedIn.
For instance, you can see me, but not anything about me because we're not in the same organization and don't have any shared boards.
We have posted an FAQ on privacy and security.