Press CTRL+SHIFT+Y/⌘+SHIFT+Y to click every 'Follow' button on the current Twitter page.

user.js

// ==UserScript==
// @name         Auto-Follow Twitter Users
// @namespace    https://gist.github.com/thosakwe/6f9fb4c3b7df5bac20c552781409dd41
// @version      1.0
// @description  Press CTRL+SHIFT+Y/⌘+SHIFT+Y to click every 'Follow' button on the current Twitter page.
// @author       thosakwe
// @match        https://*.twitter.com/*
// @require      https://code.jquery.com/jquery-3.1.1.slim.min.js
// @require      https://cdnjs.cloudflare.com/ajax/libs/mousetrap/1.6.0/mousetrap.min.js
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
    
    function loadScript(src) {
        var s = document.createElement('script');
        s.src = src;
        document.body.appendChild(s);
    }

    function mass(jQuery, options) {
      var $ = jQuery;
      var opts = $.extend({}, {
        maxButtonsInWorker: 20
      }, options);

      var queue = [];

      $(".user-actions.not-following.not-muting").each(function() {
        var group = $(this);
        var userId = group.attr("data-user-id");
        var name = group.attr("data-name");
        var btn = group.find("button.follow-button");

        queue.push(new Promise(function(resolve, reject) {
          try {
            btn.click();
            console.log('Followed user #', userId, ": ", name);
            resolve();
          } catch(e) {
            reject(e);
          }
        }));
      });

      Promise.all(queue).then(function(results) {
        alert('Followed ' + results.length + ' user(s)!');
      }).catch(function(e) {
        console.error(e);
        alert('Failed to auto-follow.');
      });
    }

    if (!window.jQuery)
        loadScript('https://code.jquery.com/jquery-3.1.1.slim.min.js');
    if (!window.Mousetrap)
        loadScript('https://cdnjs.cloudflare.com/ajax/libs/mousetrap/1.6.0/mousetrap.min.js');
    $(document).ready(function() {
        console.info('Ready to auto-follow...');
        window.Mousetrap.bind(['command+shift+y', 'ctrl+shift+y'], function() {
            if (window.confirm('Auto-follow users?')) {
                mass(window.jQuery);
            }
        });
    });
})();