Thanks for the feedback. I agree that an in-product method of contacting users, teams of users, etc would make administration much easier.
At the moment, the Sys Admin can set a message of the day, which is pinned to the top of the Dashboard activity frame. The next update we'll be putting out will add an email component to this, so users will also get an email through to bring their attention to the message. Whilst not exactly what you're looking for, I guess great minds think alike :)
A more granular approach to this, as you specify would make for a good improvement. I'll get it logged right away.
Also, just to try and help you in the short term you could have your users subscribe to the Team activity, for the Team they sit within. When a user with access to that Team activity feed posts a message, everyone who's subscribed will get an email through.