Unfortunately, there really isn't a way to do this out of the box with mojoPortal. What you could do is use the Forums module and have everyone subscribe to notifications from the forum. The problem with this approach is that it isn't very fluid for the end user. You could make it a little easier for the end user by using the Feed Manager to display the messages on the home page so the user doesn't really see the forum. Unfortunately, this means you would have to turn on the RSS feed capabilities of the forum and that will allow anyone that know the RSS feed url to see the contents of the forum.
If you decide to go with that route, let me know and I can give you more details on how to set it up.