In NEW MESSAGE window click VIEW / ALL HEADERS and enter your recipients'
names/addresses in the BCC field. The recipients will not see them. Leaving
the TO field blank may cause problems with some servers. You can enter your
own name/address in the TO field.
While composing your message click View/All Headers. Then you will see the BCC line added (that stands for "blind carbon copy"). Put the recipients - or the group name - in that line and each will receive a message addressed TO: Undisclosed Recipients.
You may have to put your own name/email address in the TO field as some ISP's require this.