Hi. This is not yet supported by K2. The Joomla! users pages define the fields of the form in XML files ( since version 2.5 ) while K2 uses direct HTML. If you do not need the K2 user profiles you can disable them in the K2 paramaters. If you do so, the default users registration page will appear ( including reCAPTCHA).
JoomlaWorks Support Team
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