Within Comet every user is a member of a nominal group called "none". There is no direct access to the "none" group so what value is there in having a group? Well to enrol, say, 30 users into a course then you'd have to go to the Users pane select Enrol users in a course select the 30 users and to then select the available course.
If you had created a group called, say, Thirty and added the 30 users to the group then you'd be saved the hassle of selecting all 30 users when enrolling them in a course. Enrolling the group automatically enrols all members of a group. Of course there's nothing to prevent you from dealing with users individually even though they are members of a group you have created.
Users can be members of any number of created groups. Though when you add users to multiple courses then you'll need to be careful that contradictory actions are not made. For example, if two groups are enrolled in the same course, un-enrolling one group would also un-enrol the members of the other group that are in the first group ("last" actions take precedence).
Groups can allow you to do some actions easier and quicker than dealing with many users individually.