Create a recordset to return the users in the users table.
You could add an HTML Editor instance to the page for the client to create the email message. But be ware that css styling from the HTML editor instance may not work in the email.
In the Universal Email instance, Set the to Address to loop through the recordset using the email address column.
For the body, set the option to create a file for the email message which automatically add the information entered in the HTML Editor instance to the email message.