Add custom item to the end of a menu

gistfile1.txt

function add_member_link_item_to_menu( $items, $args ){
  //change theme location with your them location name
  if( is_admin() || $args->theme_location != 'primary-right-menu' )
    return $items; 

	$link = '<a href="#">Member Area</a>';

  return $items .= '<li id="members-only-link" class="menu-item menu-type-link">'. $link . '</li>';
}

add_filter( 'wp_nav_menu_items', 'add_member_link_item_to_menu', 50, 2 );