Workspaces that subscribe to Trello Premium can add "Observers" to boards within their Workspace.
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, tick the box on a checklist item, and observers can't change board settings.
Because an observer is a board member permission and not a membership type, a board admin in a Premium Workspace can set both Workspace members and guests as an observer on a board.
There are no limits to how many observers you can have on a board, and an observer can be a billable Workspace member, or multi-board guest, or a non-billable single-board guest.
To understand if an Observer is billable, check out the page How billing works with Trello Premium and Standard.
Add an observer
To add someone to your board as an observer, click Share 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 dropdown to change that to 'Observer'.
You can also generate a sharable link which will add all users who use it to your board as an Observer. You can learn more about this at Inviting people by using a shareable link.
Because an observer is a board member permission it isn't currently possible to set a user as an observer at the Workspace level, and each observer is set individually per member per board.
Change a member's permissions to observer
To change a current board member’s permissions to observer, click the Share button from the top of the board, then select the drop-down next to their name and avatar and select ‘Observer’. Please note that only Workspace admins and board admins can change a board member’s status to observer.