Ok, I'll take a crack at it...

Since you are running against a remote site, you probably don't have access to errors in the server log to tell you how the POST was handled.

The html looks ok.

I am wondering if, since you are running against a remote site, you are having trouble calling the action like

<form method="post" action="my_friends_invite_user">
[download]

instead of going to the URL as in:

<form 
  method="post"
  action="http://youtube.com/my_friends_invite_user">
[download]