I have a form in a block on my side bar that I want to have it displayed to everyone however I only want the "Submit" of the form to work for registered users. If a guest clicks the submit button, I want it to go to the Register/Login page...as an incentive to get guests to register.
What is the form action? You can assert user status in your controller if it submits to a XenForo page. At least that's how it would be done in terms of XenForo code. Or you can use a template condition to only show the submit button for registered users: