How can I setup a mailing list that publishes and email to all members when logs in and presses a button on a page?

Question

I want to setup a site where users can register and then simply logon and press a button on a web page (that I will need to build) that then post a generic email to everyone. I want a button on the page that the user can press that will result in an email being sent to all members of the list/group

I am looking at mail man.

Any ideas?