How can I add a tick box for opt in?

Not quite sure if I understand your concern correctly.

When you say "opt in", are you referring to sending specific email notifications based on a tick box?

If that's the case, it is possible using the Conditional logic, you can check this guide on how to send notifications based on user inputs: http://www.jotform.com/help/167-How-to-send-to-a-specific-email-address-based-on-a-selection 

Let us know if I misunderstood your concern.