We have a couple places where we users can manually notify a group of users:

• announcing a new event
• event reminder
• mailing all participants of an event

And I think there should be another place to do that with the users filtered in the user list.

Now, I think it would be great in many cases if I could customize the message before sending and review the recipients in a separate view. All the buttons for the event notifications could still exist but would redirect to a view with subject, body and event prefilled. You could add/remove participants, customize the text and hit send.