gistfile1.rb

# Организация
class Organization < ActiveRecord::Base
  has_many :organization_members, class_name: 'Organization::Member', dependent: :destroy
  has_many :members, through: :organization_members

  attr_accessible :name
end

# Член организации
class Organization::Member < ActiveRecord::Base
  belongs_to :organization
  belongs_to :member, class_name: 'User', foreign_key: :member_id

  attr_accessible :organization, :organization_id, :member_id
end

# Пользователь
class User < ActiveRecord::Base
  has_one :organization_member, class_name: 'Organization::Member', foreign_key: :member_id
  has_one :organization, through: :organization_member
end

# Контроллер
class OrganizationsController < ApplicationController
  def create
    @organization = Organization.new(params[:organization])

    # Делаем текущего пользователя членом организации
    # !!! Вот это место мне не нравится. Хотел сделать так:
    # current_user.create_organization(params[:organization]) - но так не работает.
    if @organization.save
      current_user.create_organization_member(organization: @organization)
    end

    respond_with @organization, location: organization_path
  end
end