Go to the users channel click on send message.
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You can send a message on YouTube by using the private messaging system provided to each user. As long as a user didn't block you on YouTube, you will be able to send messages to the user you wish to send a message to. To access the private message feature, you should look for the "Inbox" link once you login to youtube when you drop-down the bar under your username on the top-right corner of any YouTube page.
Yes. You can private message them. Go to your inbox, compose, and select a username
click the username the person you want to send the message to, go to their discreptions and scroll down to send message
On the upper right hand corner of the persons wall there is now a "Send Message" icon, click on the words and it will open the standard email form.
type /r and then hit the space bar