Students do not see group calendar events, even though they are members of the group the event is assigned to. They are not visible in any of the student's calendars (course, site etc).
Discussions in forums suggest this is quite a common issue.
http://moodle.org/mod/forum/discuss.php?d=79590#p353201
http://moodle.org/mod/forum/discuss.php?d=79490