Alex Moss

Google+ comments was introduced in April 2013 to blogger.com users. This plugin lets you add Google+ comments to your WordPress site!
Simply install the plugin and follow the instructions on the Settings page.
<ul>
<li><a href="http://3doordigital.com/wordpress/plugins/google-plus-comments/" rel="nofollow ugc">Google+ Comments</a> Plugin homepage.</li>
<li>More <a href="http://3doordigital.com/wordpress/plugins/" rel="nofollow ugc">WordPress Plugins</a>.</li>
</ul>
You can also insert the comment box as a shortcode into any post, page or template and use your own settings for each time you do it! Simply use the shortcode [gp-comments]

Google+ Comments

The Google+ Comments WordPress plugin makes it easier for you to setup, administer and customise Google+ comments from your WordPress site.

Daniel Bachhuber

Easily subscribe to a P2 comment thread without commenting using a “Follow” action link like WordPress.com has.
This plugin requires Jetpack and P2 to both be activated. It doesn’t yet allow you to unfollow, and there are some minor interface quirks.
Happy to receive feedback, questions, feature suggestions, and <a href="https://github.com/danielbachhuber/Jetpack-Follow-Link-for-P2" rel="nofollow ugc">pull requests</a>

Jetpack Follow Link for P2

Easily subscribe to a P2 comment thread without commenting using a "Follow" action link like WordPress.com has

Google+ Comments vs Jetpack Follow Link for P2