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For a scenario where you could have users observing and commenting, but not changing the contents of cards, you must set the permissions a certain way.
First, the board must be Public or part of a Team. It cannot be Private.
Next, in the board settings you will have options for Commenting Permissions. I would recommend "Members and Observers" or "Team Members" for this setting.
Finally, you can modify the options so that team members cannot join the board. With members unable to join the board, they will be unable to make changes to the cards. But with the Commenting Permissions set broadly, they will still be able to make comments.
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