How do you send email to many people without everyone knowing who else is receiving it

You should use the Bcc: header. Bcc stands for "Blind carbon-copy". All mail clients, including Mozilla Thunderbird, Outlook Express and Webmail have this option.

You should leave your To: field blank, and type in all the addresses in the Bcc: field. The message will be delivered to all recipients, but they have no way of knowing who else received it.

When they reply, they can only reply to the person that sent the message and people that were listed in the To: and Cc: fields.