Trello – Is it possible to broadcast a message in Trello

You could get this behavior right now by creating an organization and inviting everyone you want to be able to vote and comment to the organization, then creating a board within the organization and making your internal users the members of the board. Then go to board preferences and make the board visibile, votable, and commentable to organization members. It's a slight abuse of organizations, but it'll get you the behavior you want.

The side effect, right now, will be that the organization's existence, and its memberships, will be public-visible. Private orgs are in test right now (https://trello.com/card/board/private-organization-profile-option/4d5ea62fd76aa1136000000c/4e7206a8ec4fdc00003e63e7) so if the organization needs to be completely invisible to the public, you would want to wait for that.

They will be unassigned from cards, and that's about it. Their actions will be preserved, as will the fact that they performed the actions.