Hi Exis,

I sincerely apologize that there's no setting available for the current system to auto subscribe.
However, you may replace the attached file at JoomlaFolder\media\com_easysocial\apps\fields\user\mailchimp\themes\default\register.php
( when users in registration page , the mailchimp will autocheck by default. )
Can you take a look and let us know about this?