Here is WP Plugin Google Plus for wordpress.
Otherwise following article may help you to add manually.
We have observed many WordPress website users/owners wants to add Google+ sharing button with their posts. And First thing you need to do is put the following code in your section of your site which you can change/alter by editing the header.php file of your wp theme.
Google+ Add to Circles Method:
[php]<script type="text/javascript">// <![CDATA[
(function()
{var po = document.createElement("script");
po.type = "text/javascript"; po.async = true;po.src = "https://apis.google.com/js/plusone.js";
var s = document.getElementsByTagName("script")[0];
s.parentNode.insertBefore(po, s);
})();
// ]]></script>[/php]
And do not forget to replace the {plusPageID} with your own Google+ Page ID. Your Page ID is a 21-digit string at the end of the URL. For example if your page URL is: https://plus.google.com/101634180904778803404/ then the numbers in bold is your Page ID.
Once you have added the header code, then you have to place the following code where you like to add your button/badge.
Most users like to display this in their sidebar, so you can either modify your sidebar.php file, or simply add it in a text widget area.
[php]
<g:plus href="https://plus.google.com/{plusPageID}" size="badge"></g:plus>
[/php]
For the Small badge, Just use the following method:
[php]<g:plus href="https://plus.google.com/101674180904808003404" size="smallbadge"></g:plus>[/php]
I hope you have got it and Now it must work on your wordpress website with lot of blessings from Google with good search rank.
