If I join a team, can everyone in the team see my private boards?

I have some private boards in Trello that I want to keep completely private. Will the members of the team I join be able to see them? I understand that I can just create new Trello accounts for each team I join, like Slack: each team has its own login and they are completely separate, but that's not convenient for me. Trello has an "Add Account" feature, and I'm wondering if I make multiple accounts, will I then be able to copy my boards and cards between different accounts?