autojoin.user.js

// ==UserScript==
// @name          G+ Hangouts Autojoin
// @description   Automatically click the join button when loading a hangout
// @include       https://plus.google.com/hangouts/_/*
// @match         https://plus.google.com/hangouts/_/*
// @version       0.1.0
// ==/UserScript==

(function(){

  function addJQuery(callback)
  {
    var script = document.createElement("script");
    script.setAttribute("src", "https://ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js");
    script.addEventListener('load', function()
    {
      var script = document.createElement("script");
      script.textContent = "(" + callback.toString() + ")();";
      document.body.appendChild(script);
    }, false);
    document.body.appendChild(script);
  }

  function checkForPrompt()
  {
    function simulate(target, evtName)
    {
      evt = document.createEvent("MouseEvents");

      evt.initMouseEvent(evtName, true, true, document.defaultView, 0, 0, 0, 0, 0, false, false, false, false, 0, target);
      target.dispatchEvent(evt);
    }

    function simulateClick(target)
    {
      simulate(target, "mouseover");
      simulate(target, "mousedown");
      simulate(target, "mouseup");
      simulate(target, "mouseout");
    }

    $('div[role="button"]').each(function(idx, item) // For each div with attribute role = "button"
    {
      if ($(item).html().indexOf("Join") >= 0) // Correct button found
      {
        simulateClick(item);
      }
      else
      {
        setTimeout(checkForPrompt, 1000); // Try again in a second
      }
    });
  }

  function init()
  {
    addJQuery(checkForPrompt);
  }

  setTimeout(init, 1000); // Start after 1 second

})();

this was eventually crashing hangouts for me since calls to setTimeout for every button that is not Join fan out exponentially, my fork seems to fix that: https://gist.github.com/ericcj/eebc7a04d6d80e93a1c9