You probably want to look at the Account Notification endpoint. That will post an announcement to the entire account, which appears on the user's dashboard.
As far as email notifications from that announcement, that's set per user. You can set the notification preferences on a user to include email, but if they haven't verified their email, it won't send. I have a Python script that does it, but it required using provisioning reports to get the correct email an a bunch of processing. Not fun, but doable.
