Someone else may know a better way, and I'm not 100% certain about this, so make sure to keep a back up of your original file(s).

Having said that, I think the easiest way is probably to use the smarty commands on your /templates/view_video.tpl

{if $smarty.session.USERNAME ne ""}

All original code goes here.

{else}

Copy all code from the Sign Up page here

{/if}

I think that will do the trick, but again, make sure to keep a copy of the original file in case you have to revert back.