The group is already set to "Sender chooses recipient" and, unless more people speak up to request otherwise, the intention is not to change it because all the other options are much more prescriptive. It does mean people need to reply-all when using emails, but the default is to post to the group when using the web interface.
On a different note, it seems Gmail has become more aggressive about filtering Google Groups email to spam in recent weeks and I have just discovered a bunch of messages in my spam folder. Apologies for that, and the upcoming flurry of emails...
Nick