Editing Group "External Users" Message Settings

I'm trying to change the Message Access Permission - Message Access Permission from "Only Specified Recipients can access" to "Only Specified Recipients and Local Users can access". But whenever I try that, I receive this error banner after clicking Save: "Please remove all users from the group before changing local".

Can you explain why I would need to remove all users and how I can accomplish this with a default External Users group?

I want to change the default setting so when a client creates an account and then creates a new message, if they forget (or don't know) to CC a secretary, that the secretary can access the uploaded files without having to involve one of the addressed parties or yours truly, the system administrator.

This was one of the groups for which I hadn't configured the default message settings, but see that I should have done so, but apparently can't after the fact.

Thanks.

Cheers,
Jon