Pokebase uses a system called Question2Answer that includes all the login stuff (but I have developed login areas before now by myself). Other programs like Wordpress or Joomla do this too. [@Will, they use PHP not Java ;)]
But Google Sites doesn't have PHP, these days you can only get that if you pay for hosting. GS might have some member management or login stuff though, I don't know. You should go and ask them.