Is it possible to add 'read only' guests, where they can see progress but not change anything?

[Trevor Hamon]

Is there a way to give the client access to their project's board as 'read only?

Or is the best option to clone the Team Board and give the client full access to the clone to allow them to add info and respond to team queries and to allow them to ask questions about the progress of their project, etc.

[Gaurav Kataria]

Trevor, you could consider adding them as Observers (available on the Premium plan and above): https://support.atlassian.com/trello/docs/adding-observers-to-boards/