Hi Poria,
Teams that subscribe to Trello Business Class can add "observers" to boards within their team. Observers are board members that are able to view a board and the cards within the board, download card attachments, comment and vote on cards if that is enabled for the board, and export the board. Observers are not able to move or edit cards, upload card attachments and observers can't change board settings.
For free teams, we suggest creating separate boards for different permissions levels and keeping certain content on the relevant boards.
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