If a group is set to allow posts by non - members any such post is placed in Pending with a badge attached to inform any passing owner or moderator that the post is from a non - member. However, that badge uses the letters NM against the standard blue background. As such, unless examined more closely, it looks very like the NMM badge so the possibility of confusion cannot be ruled out. When the post is opened for checking before release (or rejection) then the badge is expanded to read Non Member against the same colour blue background. Obviously this is rather less easy to misinterpret.
If the post passes moderation then it is not clear from either the web UI or the email sent to members that the post is from a non - member. I used my wife's email address for the test and because of the way her email is set up her correct Forename & Surname were shown as the Display Name, and the members of such a group would be unlikely to realise straight away that this was not a member. The email sent to a group also gives no clue to the poster's non - member status, or none that I have been able to detect.
Suggestion 1: Change the background colour on the NM and Non - Member badges to something other than standard blue. Something that is more clearly a "warning" (e.g. red) might be better.
Suggestion 2: Somehow flag up the fact that any given post is from a non - member for both web UI & email reading. Perhaps use a default Display Name of Non Member or a message level #hashtag advising a non - member message.
(As an aside the NM badge is not mentioned in the Owners Manual, but I'll raise that on "docs" later.)