Mark,

 Hmm. At first blush doesn't feel like it should be a #hashtag thing, but
 maybe a groupname+subgroup@groups.io thing. Hashtags are more for
 archive organization and muting.
 
 Anyone else have any thoughts on subgroups?

Well, the owners and moderators of a group are a natural example of a sub-group; one partially supported via the groupname +owner@groups.io address. The usual reason that Yahoo Group moderators create a separate group for the mods/owners is to keep an archive of the messages, and possibly files and other resources, which isn't available via the +owner email forwarding.

Another take on that idea would be to make the groupname+subgroup addresses (including +owner) fully instantiated groups, but with lowered (different) join ceremony. Specifically the +owner subgroup would come into existence automatically, and members promoted to moderator or owner role could be joined to it by virtue of that promotion.

Other subgroups could be created ad-hoc by a moderator (with that privilege) or an owner, and the moderator/owner would be able to selectively add members to the subgroup without the overhead of invitation. I don't think it should be possible to join the subgroup(s) without being a member of the root group. It could be an option in each subgroup whether members can see the existence of the subgroup; and joining could be controlled by an option with the usual three choices (any root member can join, any root member can request membership, membership by anointment only).

Hmm... there's a namespace collision problem with +subscribe, +unsubscribe or any future email commands. There are probably other problems, I haven't thought this through for longer than it took to type this.

-- Shal