diff --git a/User_Manager.php b/User_Manager.php
index 87074ef..1231a06 100644
--- a/User_Manager.php
+++ b/User_Manager.php
@@ -145,17 +145,47 @@ class User_Manager
         ));
 
     }
+    public static function del_user_from_group(PDO $db, $userId,$groupId)
+    {
+        $tableRel = self::get_table_users_groups_rel_str();
+        $sql = "DELETE FROM $tableRel WHERE user_id=:userid AND group_id=:groupid;";
+        $rs = $db->prepare($sql);
+        $rs->execute( array(
+                ":userid" => $userId,
+                ":groupid" => $groupId
+        ));
+    }
     /*
     *   Return an array of users objects.
     *   is_connected for each ones is set to false
     */
-    public static function get_users_list(PDO $db, $activesOnly = true)
+    public static function get_users_list(PDO $db, $activesOnly = true, $groups = null)
     {
         $tableUsers = self::get_table_users_str();
         $tableGroups = self::get_table_groups_str();
         $tableRel = $tableUsers . "_" . $tableGroups ."_rel";
 
-        $list = array();
+        //conditions
+        $conditions = array();
+        if($activesOnly){
+            $conditions[] = "$tableUsers.active='1'";
+        }
+        if(!is_null($groups)){
+            //$ groups is an untrusted entry; check it before using it on a non a query
+            foreach($groups as $group){
+                if (!preg_match('/^[0-9]+$/',$group)){
+                    throw new \UnexpectedValueException('$groups must be a list  containing only digits'. $group);
+                }
+            }
+            $conditions[] = "$tableUsers.id IN(
+                                                    SELECT $tableUsers.id
+                                                    FROM $tableUsers, $tableRel
+                                                    WHERE $tableRel.user_id = $tableUsers.id
+                                                    AND $tableRel.group_id IN ('" . implode("','",$groups) ."')
+                                                )";      
+        }
+
+        
 
         $sql = "
             SELECT 
@@ -170,12 +200,13 @@ class User_Manager
                 GROUP_CONCAT(groups.name SEPARATOR \",\") as groups_name
             FROM
                 $tableUsers LEFT JOIN  $tableRel as groupsrel ON groupsrel.user_id = $tableUsers.id
-                    LEFT JOIN $tableGroups as groups ON groups.id = groupsrel.group_id
-            GROUP BY $tableUsers.id;
-        ";
+                    LEFT JOIN $tableGroups as groups ON groups.id = groupsrel.group_id"
+            .(empty($conditions) ? "" :  " WHERE " . implode(" AND ", $conditions))
+            ." GROUP BY $tableUsers.id; ";
       
         $rs = $db->query($sql);
-        
+    
+        $list = array();
         while ($r = $rs->fetch())
         {
 
@@ -204,7 +235,4 @@ class User_Manager
         }
         return $list;
     }
-
-
-    
 }
\ No newline at end of file