Hi there - I split your post from the earlier one as it is more specific. I tried on our School demo site Mount Orange here http://school.demo.moodle.net and couldn't replicate it
Could you try please?
Log in to http://school.demo.moodle.net with the username teacher and password moodle and then go to the Psych Cine (Psychology in Cinema) course here http://school.demo.moodle.net/course/view.php?id=62 because it does have three groups - albeit small. Try messaging one group and previewing.