Colin,
Is it possible to configure the system in some way to allow my
members to email this subgroup without being part of the subgroup
itself.
I have a similar situation with the board subgroup to my PTA group. Unit members (members of the primary group) ought to be able to email the board, and just the board, but shouldn't receive other's messages to the board nor messages among the board members.
My solution is the same as yours, but my traffic is low enough that the moderation of messages to the board hasn't really been a burden.
The idea has been mentioned in beta@, but not presented as a feature request in its own topic. If you would like to start such a topic I'll gladly second it.
Or a way to add them to this subgroup so they can email freely,
without them receiving all emails other than their own?
No.
At least, not in an enforceable way. You could set them to no mail, but they'd still be free to peruse the subgroup's messages on site, and could change their email subscription setting at any time.
Shal