You'll want to make sure you're calling 866-623-7525, and you'll enter in your User ID and PIN that you use to log into the website.
Instead of pressing 1 to send to everyone, you'll press 2 to send it to a specific group. Then, you'll enter in the List ID number of the group.
You'll need to know the List ID of the group that you'll be calling ahead of time. You can find this in your Contact list by dragging your mouse over the group name.
You'll see it above the list of names & numbers as well:
Then, record your message, and press 1 to accept it!