I've modified project_api.php (find it attached to this report) in the following way:

function project_get_all_user_rows( $p_project_id = ALL_PROJECTS, $p_access_level = ANYBODY ) {
....
....
$t_adm = ADMINISTRATOR;
$t_users = array();

  if( $c_project_id == ALL_PROJECTS ) {
      $t_user_id = auth_get_current_user_id();

      $query = "SELECT u.id, u.username, u.realname, u.access_level
      FROM $t_user_table u, $t_project_user_list_table l
        WHERE u.enabled = $t_on
          AND u.access_level $t_global_access_clause
          AND l.user_id=u.id
          AND (l.project_id in (select project_id from
          $t_project_user_list_table where user_id = $t_user_id ))";
     } else {
          $query = "SELECT id, username, realname, access_level
              FROM $t_user_table
              WHERE enabled = $t_on
             AND access_level $t_global_access_clause";
             }

             $result = db_query( $query );

....
.....
}

This works well in my configuration. I'm not sure yet, if the change interferes some other functionality.