Acredito que exista uma solução melhor do que reescrever o RegistrationsController. Fiz exatamente a mesma coisa (apenas tenho organização em vez de empresa).
Se você definir corretamente seu formulário aninhado, no nível de modelo e exibição, tudo funcionará como um encanto.
Meu modelo de usuário:
class User < ActiveRecord::Base
# Include default devise modules. Others available are:
# :token_authenticatable, :confirmable, :lockable and :timeoutable
devise :database_authenticatable, :registerable,
:recoverable, :rememberable, :trackable, :validatable
has_many :owned_organizations, :class_name => 'Organization', :foreign_key => :owner_id
has_many :organization_memberships
has_many :organizations, :through => :organization_memberships
# Setup accessible (or protected) attributes for your model
attr_accessible :email, :password, :password_confirmation, :remember_me, :name, :username, :owned_organizations_attributes
accepts_nested_attributes_for :owned_organizations
...
end
Meu modelo de organização:
class Organization < ActiveRecord::Base
belongs_to :owner, :class_name => 'User'
has_many :organization_memberships
has_many :users, :through => :organization_memberships
has_many :contracts
attr_accessor :plan_name
after_create :set_owner_membership, :set_contract
...
end
Minha opinião: 'devise / registrations / new.html.erb'
<h2>Sign up</h2>
<% resource.owned_organizations.build if resource.owned_organizations.empty? %>
<%= form_for(resource, :as => resource_name, :url => registration_path(resource_name)) do |f| %>
<%= devise_error_messages! %>
<p><%= f.label :name %><br />
<%= f.text_field :name %></p>
<p><%= f.label :email %><br />
<%= f.text_field :email %></p>
<p><%= f.label :username %><br />
<%= f.text_field :username %></p>
<p><%= f.label :password %><br />
<%= f.password_field :password %></p>
<p><%= f.label :password_confirmation %><br />
<%= f.password_field :password_confirmation %></p>
<%= f.fields_for :owned_organizations do |organization_form| %>
<p><%= organization_form.label :name %><br />
<%= organization_form.text_field :name %></p>
<p><%= organization_form.label :subdomain %><br />
<%= organization_form.text_field :subdomain %></p>
<%= organization_form.hidden_field :plan_name, :value => params[:plan] %>
<% end %>
<p><%= f.submit "Sign up" %></p>
<% end %>
<%= render :partial => "devise/shared/links" %>