I think it's a good way to allow non-members to follow along with commons activity and a nice bit of functionality that will help keep more outward facing groups on the commons.
Like Boone I think more many-many with listserv where anyone can send messages to all members, but Matt you named it right later it is a way to subscribe to a group, aka a mailing list (email list / subscription list).
I'll have to think about it a bit more from a UX perspective. People who are Commons members can already get this functionality by just joining the group. So this is for people who are not members and can't join (or for some reason don't want to join but want to get messages). We'll have to think of a way to make that distinction clear somehow to people so members know they should just join the group and non-members know what's available to them. I'm guessing that the option won't show if you're logged in.
Also, in terms of functionality are we just going to limit it to announcements or do we want to allow them to get emails on other things like forum posts? And do they get the same kind of control over when and what emails to get? That would be kind of like joining the group in a read-only or subscriber mode.
Either way there will also need to be a way that people can request to stop receiving the emails at some point. Boone, you may have more ideas on this but to me that means there either needs to be 1) a link in every email sent this way with an unsubscribe option, 2)some kind of login. Number 2 sounds like a lot of work unless we make it part of the work to allow membership in a limited capacity to the Commons (the stuff we are thinking about regarding non-cuny folks joining groups) and have a subscriber status that anyone in the world can opt into (still sounds like a lot to think through in that option).
On a slight tangent it's making me wonder if this kind of functionality means we need to add something in help for people who are non-members on what they can and can't do on the commons.