OK, an update
I finally got Discourse Automation to work - the trick was to use a drop down custom user field type (even though the instructions don’t spell this out) rather than the tick box field type I had started with. The drop down options need to correspond exactly to the full user group names
Very important - ensure this new field is not user-editable after sign up, otherwise a user signing up and being approved only for the public forum can later just unilaterally grant themselves themselves acess to the private forum.
