Populating one form from another on the same page

Hi,
I read through this post but I didnt quite figure it out:
http://www.gravityhelp.com/forums/topic/populating-from-a-previous-form

On this page:
http://launch0812pre.stablish.me/
I have 4 Forms: 1x Register your email (header), 1x Register your email (footer), and 2x invite friends (header/footer)
After you register your email, you should be able to invite 5 friends.
My problem is, I donno who filled out the second form.
I would like the invite friends form to have a hidden field, that is populated from the input in the first form (emailadress of visitor), but without a page reload. Like this I should be able to see who invited whom...
I guess this cannot be 2 hard, since all the forms are already on the screen (display:none) when the page is loaded. I think the solution would be some JS, but I have no idea how or where to start. Any help, or pointers in the right direction are appreciated.

This is what I was looking for:

[js]
		$('#input_5_1').keyup(function () {
			$('#input_4_6').val(this.value);
		});
		$('#input_6_1').keyup(function () {
			$('#input_7_6').val(this.value);
		});

Hope this helps someone else. It populates the field 6 in form 4 from the value in form 5 field 1, and the same for form 6 and 7.