Use Announcements mechanism to send a message either to all users or to selected users and groups only.
Go to Feed and type your message:
Make sure to choose recipients of the message:
Selected users and members of groups you pick will receive email notification with the content you typed and the same message will be displayed in their Feed.
Only those user who have permission Publish news to the feed will be able to send these messages:
This privilege is automatically enabled for Administrator role, make sure to enable it for other roles if necessary.