(My apologies if this has been mentioned before--I couldn't find a post to this topic) It would make us deliriously happy if there was a way to email a user group (not just individual members), as you can agency fans, etc. Would GD consider adding that filter to in-app messaging?
The email blast currently has that filter (User Data > User Group > belongs to), but you are correct, it is not currently available in in-app messaging. The good news is that all emails are also sent to the in-app messaging inbox. The main downside here is that, if you want to use in-app messaging only and not the email blast, the user group option isn't available.
Are you specifically asking that it be added for in-app messaging only? I'll be happy to add that to our enhancements discussion list.