A userscript to set the current Google account (according to `?authuser=`) when joining and leaving tests.

GooglePlayTesting.user.js

// ==UserScript==
// @name         Google Play Testing per-account
// @namespace    http://terrance.uk.to
// @version      0.1
// @description  Makes the "Become a Tester" button respect the current auth user.
// @author       Ollie Terrance
// @match        https://play.google.com/apps/testing/*
// @grant        none
// ==/UserScript==

(function() {
    var au = location.search.match(/authuser=([0-9])/);
    if (au) {
        var jf = document.getElementsByClassName("joinForm");
        if (jf.length) jf[0].action += "?" + au[0];
        var lf = document.getElementsByClassName("leaveForm");
        if (lf.length) lf[0].action += "?" + au[0];
        var as = document.getElementsByTagName("a");
        for (var i in as) {
            if (as[i].href && as[i].href.match(/details\?id=/)) as[i].href += "&" + au[0];
        }
    }
})();