Starting version 1.16.1 in Theme Options you can enable or disable reCaptcha for register form. This is a useful tool to prevent registration spam.

recaptcha

To use this feature you must add site key and secret key which you take from this url https://www.google.com/recaptcha/intro/index.html 

If you get this error

Recaptcha: ERROR for site owner: Invalid domain for site key

Make sure the site url is added here