Skysa

Simply set the Twitter account you want users to follow; then, when they click the Twitter Follow button at the bottom of your site, they will start following your account on Twitter. You can optionally set to display your current followers count next to the Twitter follow button and also the language of the Twitter follow button text.
http://www.youtube.com/watch?v=Zj1uGXX2xrk&hd=1 
<a href="https://wordpress.org/extend/plugins/tags/skysa-apps" rel="ugc">More Skysa App plugins</a> -|||- <a href="https://wordpress.org/extend/plugins/skysa-official/" rel="ugc">Skysa App Bar Integration plugin</a>

Skysa Twitter Follow App

Allows your site visitors to follow your twitter account with one click, without leaving your sitet.

Michal Ozogan

This plugin will allow visitor to submit they twitter, while thay are commenting your posts. They account will by showed as twitter follow button.
Registred users can add their twitter account in profile administration.
There is also a setting page where you can edit color, size, showing count and language.
Some themes dont’t use function <code><?php comment_form(); ?></code> to show comment form. In this case you have to edit your template. Find comments.php and add
<pre><code><label for="twitter">Twitter</label><input name="twitter" size="30" type="text" value=''/>
</code></pre>
to form.

Twitter follow button in coments

Allow your visitors to add their twitter.

Skysa Twitter Follow App vs Twitter follow button in coments