You might consider creating an announcement in a place which would allow you to Send Inbox notifications. Another option would be to create a thread in a place that they are members of, or create a thread with specific people:
I should mention that external sites have user containers disabled by default therefore you would not have the option to create it with specific people. However this option is configurable, if you or your Community Manager agree to the change, the system property to allow it are the following I've provided. The one specific to threads is jive.usercontainer.thread.enabled = true
For this and other system properties, please see Undocumented Jive System Properties
Hope this helps.
Is there any way you can customize the text to pretty up the content? Curious