View:
<% form_for(:user, :url => domain_users_path) do |f| %>
    <fieldset>
			<label class="field-username">new email &#42;<input type="text" name="username" value="" /></label><span class="secondary_label">@<%=h @domain.domain %></span>
			<label class="field-password">password &#42;<input type="password" name="password" value="" /></label>
			<label class="field-name">full name &#42;<input type="text" name="first_name" value="" /></label>
			<%= submit_tag "Create" %>
		</fieldset>
<% end %>


controller:
  def create
    @user = User.new(params[:user])
    @domain = Domain.find params[:domain_id]
    respond_to do |format|
      if @user.save
        flash[:notice] = 'User was successfully created.'
        format.html { redirect_to domain_url(@domain.id) }
      else
        format.html { render :action => "new" }
      end
    end
  end

model:
class User < ActiveRecord::Base
  belongs_to :domain

  def before_save
    self.homedir = "/home/vmail"
    self.quota = "0"
    self.maildir = "/home/vmail/#{@domain.id}/#{:username}/.maildir/"
    :full_email
  end

  attr_accessor :username

  def password=(p)
    self.clear = p
  end
  def password
    clear
  end

  def full_email=(username)
    self.email = "#{username}@#{@domain.id}"
  end
  def full_email
    email
  end

end