In your campaign add a new Custom Entry method (either required or as a bonus entry).
Follow us on Google+<br>
<div class=”g-follow” data-annotation=”bubble” data-height=”20″ data-href=”https://plus.google.com/+promosimple” data-rel=”publisher”></div>
Your Google+ Follow button will show within your giveaway’s entry form and users can subscribe while they enter the promotion. You can also use Google’s developers section to create alternative versions of the Google+ Follow button with various widths, heights and select whether or not to display the number of followers that the page currently has.