@Paul Foster if you have a free Trello team, you can still do this by setting your Board permissions so that anyone in the team can view, disabling self-join but allowing team members to comment:
The only limitation is that you won't be able to have more than 10 boards in your free team.
You can read more about Trello board level permissions here:
Hello Community! My name is Brittany Joiner and I am a Trello enthusiast and Atlassian Community Leader. I'll be sharing with you a series of videos with my top tips on how to best use Trello...
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