Within the overview of groups (Admin block -> groups -> overview tab) there is always printed in "user count" 1 even if the group has no group member. If there are more than one member the counting is correct.
groups.png shows the group with no members, and ovreview.png shows the overview of groups and groupings with user count = 1!!!
You can look at this issue even on the demo.moodle.org site...if groups are allowed within the demo course.