Index: lib.php
===================================================================
--- lib.php	(revision 848)
+++ lib.php	(working copy)
@@ -2831,7 +2831,15 @@
         $sql = "$sql ORDER BY u.lastname ASC, u.firstname ASC";
     }
 
-    return $DB->get_records_sql($sql, $params);
+    $results = $DB->get_records_sql($sql, $params);
+    $cm = $DB->get_record('course_modules',array('id'=>$forumcontext->instanceid));
+    if ($cm->groupingid != 0) {
+        $members = groups_get_grouping_members($cm->groupingid,'u.id');
+        foreach (array_keys($results) as $userid) {
+            if (!array_key_exists($userid, $members)) unset($results[$userid]);
+        }
+    }
+    return $results;
 }
 
 /**