Automatically refresh the page until the Google Meet has started and then automatically join.

meet_autojoin.user.js

// ==UserScript==
// @name         Google Meet Auto-Join
// @namespace    http://meet.google.com/
// @version      0.1
// @description  Automatically refresh the page until the Google Meet has started and then automatically join.
// @author       ZeevoX
// @match        https://meet.google.com/*
// @grant        window.close
// ==/UserScript==

(function() {
    'use strict';

    setInterval(() => {
        let willRefresh = false;

        document.querySelectorAll("button, *[role='button']").forEach(button => {
            switch(button.textContent) {
                case "Join now":
                    if (document.hidden) alert("Google Meet ready!");
                    button.dispatchEvent(new MouseEvent("click",{bubbles: true, cancellable: true}));;
                    document.querySelector("div[data-tooltip='Leave call']").onclick(() => window.close());
                    break;
                case "Rejoin":
                    window.close();
                    break;
                case "Return to home screen":
                case "Reload":
                    if (!willRefresh) {
                        willRefresh = true;
                        setTimeout(() => location.reload(), 1000 + 2000 * Math.random());
                    }
                    break;
            }
        });
    }, 2000);
})();