@Samantha Isin‚ As a System Admin, you can utilize the Announcement Center to send in-app messages to all users. By default, when sending a new announcement message, Everyone is pre-populated in the "Send to" field, however, you can adjust that field as needed to include groups, teams, companies or specific individuals.
Unless a user has toggled off the notification for "A message is sent to the Announcement Center," an email will also be sent. The article, View and Send Announcements provides step-by-step instructions on how to do this. If you have any questions, let me know!