Adding observers to boards
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.
Add an observer
To add someone to your board as an Observer, click "Invite" from the top of the board. Enter the member's username or email address. When you choose the person to add, the default choice is to add them as regular board members, but you can click on the down arrow to change that to "Add as Observers."
Change a member's permissions to Observer
To change a current board member's permissions to Observer, click their avatar from the Members section of the board menu, select "Change Permissions" and select "Observer". Please note that only team admins and board admins can change a board member's permissions.
Change a board member's permissions to Observer.