Ability to tag other idea portal users in a given idea so that they can comment Merged

As an admin or as an idea submitter, I want to tag other idea portal users who are not subscribed to the idea. Use case: User A submits and idea, I need to comment and notify user B in the idea portal users so they can clarify something. 

This would be super basic amongst users of a given domain name such as my colleagues. I can see why you may want to limit it for outsiders initially