Arc Acorn discovered that Google Groups has several permission settings that I didn't know about. I've now made the following configuration changes to both of these discussion groups:
! Posting Permissions
Change "Add references" from "None" to "All members of group". I've no idea what "references" are but thought they may be worth exploring
Change "Post Announcements" from "None" to "All members of group". Might be useful (we may want to restrict it to managers, I guess)
Change "Me Too Topic" from "None" to "All members of group".
Change "Post Rich Text Format" from "None" to "All members of group".
! Moderation Permissions
Change "Edit Own Posts" from "None" to "All members of group"
Change "Mark Duplicate" from "None" to "All members of group"
Change "Mark Favorite Reply On Any Other User's Topics" from "None" to "All members of group"
Let me know how it goes,
Best wishes
Jeremy