a.) Ad-Hoc mailing lists / discussion groups you can subscribe to or drop out of at any time. b.) Discussions and status updates you can merely follow (and save for later) instead of participate in without those updates clogging your inbox.
The use case for both would be situations where - especially in larger companies - lots of people get CCed or BCCed because the just need to be in the loop but don't actually have to contribute anything.
I very much like you hashtag approach. It seems like the right solution to this problem. However, I don't quite understand why you took a mobile-first approach. At least in settings I have in mind a web or desktop app would make a lot more sense.
If you'd like to discuss this in more detail I'm looking forward to an eMail from you (contact form / contact details under http://wilmsmann.fullmontymedia.com/ ).