You can message all users assigned to an event by sending them a note from the event via the notification method selected by the user assigned to the event. 

The Message Assignees feature can be accessed from the Event Information screen.  

Clicking on the “Message Assignees” button opens a small text box where you can type your note to the assignees, or select one of the template notes from the drop-down menu. 

If you select one of the preset templates, the event type and description will be automatically filled in.

The templates for Message Assignee notes are customizable by Admin and can also be edited if permission is granted to any user.  The templates can be customized in the Events tab of the Data Properties section of Account Administration.