The general mailing list for dance announcements corresponds to the membership of the "contracola" google group, i.e., firstname.lastname@example.org. We are careful to send to this group using the contracola gmail account (email@example.com), so that we can tell people what email address will be sending them reminders.
So, to send an announcement to the mailing list, simply log into the contracola account (firstname.lastname@example.org) at http://mail.google.com (or http://gmail.com) and compose an email to email@example.com.
An easy way to send a reminder for the next dance is to copy the dance announcement (including the picture, if any) from www.contracola.org and paste it into the email to be sent to the mailing list. If you include the blog announcement title in the message body, it becomes a link to the website. You can also use the title as the subject of the email.