Depends on what email client you use, but, somewhere below the “To” field, there should be fields named CC and BCC, or links which open those fields. CC, of course, is Carbon Copy, letting you send a message to secondary recipients. BCC is BLIND Carbon Copy. Any addresses entered into the BCC field will not display to the recipients of the message, they will not know who the others are or be able to contact them.
OK, I assume you use the newest version of Yahoo! Mail, I just checked, the “Show BCC” link is to the right of the “To” field. In older versions of Yahoo! Mail, the Show/Remove links for CC and BCC are above the “To” field. Not sure if it requires a recipient in the main “To” field, but if it does, you can just send it to yourself and BCC the rest.

Now, if you mean you don’t even want them to be able to reply to YOU, that is another matter. You will have to use tools to spoof nonexistant from and reply-to addresses. There are several tools on the internet that can do that.

Okay, assuming that you use Yahoo! Mail Beta, you go to Show BCC and add all the e-mail addresses and then it says please specify at least one recipient in the To field. So just e-mail it to yourself and then delete it or whatever.