There is a problem with the code where it will not display users of a group if they are additional group members (the group is not their primary group).
You can fix it by finding this code around line 240 of staff.php:
Code:
LEFT JOIN " . TABLE_PREFIX . "usergroup AS usergroup ON(usergroup.usergroupid = user.usergroupid)
And changing it to:
Code:
LEFT JOIN " . TABLE_PREFIX . "usergroup AS usergroup ON(usergroup.usergroupid = user.usergroupid) OR FIND_IN_SET(usergroup.usergroupid, user.membergroupids)