This tip is very simple. Create a new sub tab and use the option to go to a URL. You can have the text be something like "You Tube" under the invite main tab or just create a button on the home page.
The secret sauce is the link. http://youtube.com/address_book. Any you tube user including yourself gets taken right to their you tube addressbook where they can easily compose a message and select their friends. This is equivalent to create a link that opens up someone's email program so they can send a "Tell a friend" email
In fact, if you are signed in to you tube you can try it now. Invite friends on You Tube.