Creating Accounts with Subusers

In my model, I have Account has_many Users. I'm using
restful_authentication, so I have the following routes and resources
defined:

map.resources :accounts
map.resources :users
map.resource :session, :controller => 'session'
map.login '/login', :controller => 'session', :action => 'new'
map.logout '/logout', :controller => 'session', :action => 'destroy'
map.signup '/signup', :controller => 'accounts', :action => 'index'
map.signup '/signup/:plan_type', :controller => 'accounts', :action =>
'new'

In my AccountsController#create action, I create both the new account
and its 'administrative' user:

def create
  @account = Account.new(params[:account])
  @account.pricing_plan = PricingPlan.find_by_name(params[:plan_type])
  @account.users[0].is_admin = true

  respond_to do |format|
    if @account.save
      flash[:notice] = 'Account was successfully created.'
      format.html { redirect_to(@account) }
      format.xml { render :xml => @account, :status => :created,
:location => @account }
    else
      format.html { render :action => "new" }
      format.xml { render :xml => @account.errors, :status =>
:unprocessable_entity }
    end
  end
end

I would like the ability for this administrative user to create
additional users under the same account. What is the best approach for
this?

Thanks,
Mark