In your ga.js (google analytics) file make a function like:
function trackYourClick(){
ga('send', 'event', 'ClickedLink', 'click')
}
Put in your link: onclick="trackYourClick()"
in your case:
<a href="#" class="reg" onClick="joinpage.style.display = 'block';trackYourClick();return false;">free account</a>