Form for Multiple Models

I have a signup form that needs to do these three things:

  1.   create a new "Group"
    
  2.   create a new "User"
    
  3.   Add that User to the Group
    

I have the scaffolding for the create new user form:

<% form_for :user do |form| %>

    <label for="user_name">First Name:</label>

    <%= form.text_field :first_name, :size => 40 %>

  </p>

  # ...

<% end %>

I want to add a “Group Name” field to this same form so that the
controller can use it to create a new Group in the database and then do
the typical @user.save, etc…