Groups and Audiences have 2 separate uses
Users need to subscribe to groups, when you create a group, there is a button for "Get signup Form HTML"
this is a signup form to add to your page to allow a user to signup to be part of that group.
Audiences allow you to define criteria for who the mailing goes to. For example you could use the UserState column to send the email to all registered users in Florida
if you are creating groups like PR committee and Financing Committee, you will need to add the signup code to a page on your site and tell Mary Jones to visit that page to signup for those groups