According to a recent study by Aweber, simply adding a subscriber count to your website can increase your opt-in rates significantly (in some cases by 32.4%) .

This increase in subscriptions is contributed to social proof.  Social proof, also known as informational social influence, is the psychological phenomenon that happens when a decision is influenced by the decisions of others.  In short, more people subscribed when they were exposed to proof that others were subscribing too.

Simply adding a statement which reads “X number of members and growing” can help increase your conversion numbers dramatically.

You can read more about the Aweber case study here:

http://www.aweber.com/blog/case-studies/social-proof-boost-landing-page-conversion.htm