RobloxAutoJoin - A TamperMonkey extension that automatically joins an user if they're playing. You must be on a profile page for the extension to be executed.

RobloxAutoJoin.js

// ==UserScript==
// @name         RobloxAutoJoin
// @version      1.0
// @description  Automatically joins an user if they're playing.
// @author       luuccss
// @match        https://www.roblox.com/users/*/profile
// ==/UserScript==

(function() {
    const userId = Number(location.href.replace(/\D/g, ""));
    let main = document.createElement('div');
    let notifications = false;
    let lastGame = null;
    let running = false;
    let autostart = [];
    let userName;
    let status;
    document.getElementById("profile-header-container").appendChild(main);
    document.cookie.split("; ").forEach(pair => {
        let [key, value] = pair.split("=");
        if (key == "autostart") {
            autostart = value.split(",").map(Number);
        };
    });
    function askNotificationPermission() {
        if (!("Notification" in window)) {
            console.log("This browser does not support notifications.");
            return false;
        };
        if (Notification.permission === "granted") {
            return true;
        } else if (Notification.permission === "none" || Notification.permission === "default") {
            Notification.requestPermission().then((permission) => {
                if (permission === "granted") {
                    return true;
                } else {
                    return false;
                };
            });
        } else {
            return false;
        };
    };
    notifications = askNotificationPermission();
    function setStatus(text, color) {
        if (!status) {
            status = document.createElement('span');
            main.appendChild(status);
        };
        status.innerHTML = text;
        status.style.color = ["#f74b52", "#00b06f", "#f7d74b", "#ffffff"][color];
    };
    function join() {
        $.ajax({
            type: "POST",
            url: "https://presence.roblox.com/v1/presence/users",
            data: {"userIds": [userId]},
            success: function(data) {
                if (data["userPresences"] && data.userPresences[0]) {
                    if (data.userPresences[0]["userPresenceType"] && data.userPresences[0].userPresenceType === 2) {
                        if (data.userPresences[0]["gameId"]) {
                            if (data.userPresences[0].gameId != lastGame) {
                                lastGame = data.userPresences[0].gameId;
                                Roblox.GameLauncher.followPlayerIntoGame(userId);
                                setStatus(`<a href='#' onclick='Roblox.GameLauncher.followPlayerIntoGame(userId);'>Joined ${data.userPresences[0]["lastLocation"]} (${data.userPresences[0].gameId})!</a>`, 1);
                                if (notifications) {
                                    new Notification("Joined a game", { body: `${userName} joined ${data.userPresences[0]["lastLocation"]}.` });
                                } else {
                                    notifications = askNotificationPermission();
                                };
                            };
                        } else {
                            setStatus(`You don't have permission to join this user!`, 0);
                        };
                    } else {
                        setStatus(`User is not playing.`, 2);
                        if (lastGame) {
                            lastGame = null;
                            if (notifications) {
                                new Notification("Left the game", { body: `${userName} left the current game.` });
                            } else {
                                notifications = askNotificationPermission();
                            };
                        };
                    };
                } else {
                    setStatus(`Error! Check console for details.`, 0);
                    console.warn(data);
                };
            },
            complete: function() {
                running ? join() : setStatus('', 3);
            }
        });
    };
    $.ajax({
        type: "GET",
        url: "https://users.roblox.com/v1/users/authenticated",
        success: function(data) {
            if (data["id"]) {
                if (data.id != userId) {
                    $.ajax({
                        type: "GET",
                        url: `https://users.roblox.com/v1/users/${userId}`,
                        success: function(data) {
                            if (data["displayName"] && data["name"]) {
                                userName = `${data.displayName} (@${data.name})`;
                            };
                        }
                    });
                    let joinbtn = document.createElement('button')
                    joinbtn.type = "button"
                    joinbtn.style = "margin-right: 15px;"
                    joinbtn.innerHTML = "Attempt to join game"
                    joinbtn.setAttribute("class", "btn-growth-md")
                    joinbtn.addEventListener("click", function() {
                        if (running) {
                            running = false;
                            joinbtn.type = "button"
                            joinbtn.style = "margin-right: 15px;"
                            joinbtn.innerHTML = "Attempt to join game"
                            joinbtn.setAttribute("class", "btn-growth-md")
                        } else {
                            running = true;
                            joinbtn.type = "button"
                            joinbtn.style = "margin-right: 15px;"
                            joinbtn.innerHTML = "Stop attempting to join game"
                            joinbtn.setAttribute("class", "btn-alert-md")
                            join();
                        };
                    });
                    main.appendChild(joinbtn);
                    let addautostart = document.createElement('button')
                    function checkifautostart() {
                        if (autostart && autostart.includes(userId)) {
                            addautostart.type = "button"
                            addautostart.style = "margin-right: 15px;"
                            addautostart.innerHTML = "Remove from autostart"
                            addautostart.setAttribute("class", "btn-alert-md")
                        } else {
                            addautostart.type = "button"
                            addautostart.style = "margin-right: 15px;"
                            addautostart.innerHTML = "Add to autostart"
                            addautostart.setAttribute("class", "btn-primary-md")
                        };
                    };
                    checkifautostart();
                    addautostart.addEventListener("click", function() {
                        if (autostart && autostart.includes(userId)) {
                            autostart.splice(autostart.indexOf(userId), 1);
                        } else {
                            autostart.push(userId);
                            if (!running) {
                                running = true;
                                joinbtn.type = "button"
                                joinbtn.style = "margin-right: 15px;"
                                joinbtn.innerHTML = "Stop attempting to join game"
                                joinbtn.setAttribute("class", "btn-alert-md")
                                join();
                            };
                        };
                        const date = new Date();
                        date.setTime(date.getTime() + 34560000000);
                        document.cookie = `autostart=${autostart.join(',')}; expires=${date.toUTCString()}; path=/`;
                        checkifautostart();
                    });
                    main.appendChild(addautostart);
                    if (autostart && autostart.includes(userId)) {
                        running = true;
                        joinbtn.type = "button"
                        joinbtn.style = "margin-right: 15px;"
                        joinbtn.innerHTML = "Stop attempting to join game"
                        joinbtn.setAttribute("class", "btn-alert-md")
                        join();
                    };
                } else {
                    setStatus(`This is your profile.`, 3);
                };
            } else {
                setStatus(`Error! Check console for details.`, 0);
                console.warn(data);
            };
        }
    });
})();