Hi Rebecca

Some options for you to consider

• Filtering by the existing audience use bulk user actions to  send a message to users - this is a pretty manual but allows messages to be customised for each group
• Use HR Import to create users - leave the password field empty and a welcome email will be sent - this is able to be changed with language customisation
• Enrol the audience members in a course - subscribe all users to announcement forum 
• Enrol the users on a program and change the program enrolment message 

Regards