2) A bulk Direct-Add ability. Rather than checking hundreds of boxes (and double-checking that I got just the right ones) it would be easier for me to copy a column of email addresses from excel to identify the members who signed up for the additional activity represented by the subgroup.
Never mind.
It turns out I can do this with Direct Add on the primary group. I get an error for each member who's already in the primary group, but they do get added to the subgroup checkmarked on the Direct Add page.
And as a bonus, from my point of view, they don't get the "You've been added", presumably because of the error on the primary group add.
But I as moderator still get the notification: "New Subscriber To subgroup@example.groups.io"