In the file block_online_users.php, there are lines for seperate groups,codes are:
//Calculate if we are in separate groups
$isseparategroups = ($COURSE->groupmode == SEPARATEGROUPS
&& $COURSE->groupmodeforce
&& !has_capability('moodle/site:accessallgroups', $context));
What is the seperate groups,what means in separate groups?
This might be a good place to search or ask your question http://moodle.org/mod/forum/view.php?id=1490.