You don't send attachments by Youtube, you upload the files to Youtube, decide whether you want them to public or private. Youtube will then tell you the URL of your video, which is all you need to send your friend. Your friend clicks the URL (or types it into his/her browser), and watches it on Youtube.
This has a number of advantages:
You don't clog up your friend's inbox or exceed their quota
They don't have to download it as an e-mail, but can watch it at their convenience
You can send the URL (which is a few bytes) to all your friends without clogging up the network.
Just go to
http://uk.youtube.com/index
Click Upload/Video File.
If you're not already signed up, you will be asked to sign up - just fill in the form and away you go.
Of course, it's all free!