OK this isn't strictly speaking a teaching tip, although you can apply it as a teacher in your course if you want a message for guests only, but it is a non-technical tip, involving no coding or hacking files on the server - the kind I like best.
There are various complex ways and theming ways to get your front page looking different for guests and logged in users. You can also use block permissions to allow only logged in users to view blocks - see Blocks FAQ - but you can't do this with a label as there is no "view label" permission. You can use Conditional activities however, if they are enabled on your site.
Here is my 21st Century Cats site (which I made earlier!) I restricted all the blocks so they are only visible to logged in users and I added a welcome label to guests:
Then, I set the label's access restrictions so:
So basically - if you don't have an email address then you can see this label. If you are not logged in, then Moodle does't know whether you have an email or not, so you can see it.
Here is the logged in view. Bobby Black can now see his regular Administration and Navigation block - but the message (label) has changed:
(By the way; I am not promoting this as good front page design - it's just the tip, not the appearance ) He can't see the previous one because as a logged in user, he has an email address - so it has gone.
In this second label, the message to logged in users, I set the access restrictions so:
It was a forum thread from the French Community that led to this idea, so Merci to Patrick Lemaire and the others over there.