Great video from Mary about site wide groups and works well in conjuction with this article on Moodle.org - http://docs.moodle.org/22/en/Cohorts and this one on synching: http://docs.moodle.org/23/en/Cohort_sync
We set up Cohorts for Moodle and Mahara training to make it easier to move groups between courses. What's more, we have also create a range of fun and popular users to our course which those attending can randomly pick from a generator like http://primaryschoolict.com/random-name-selector/ - its a great ice-breaker!
Here were the users from yesterday - can you spot a few famous ones? https://dl.dropboxusercontent.com/u/7336564/cohorts.png