gistfile1.rb

class Account < ActiveRecord::Base
  has_many :memberships, :class_name => 'AccountAdministrator'
  has_many :administrators, :through => :memberships
end

class Administrator < ActiveRecord::Base
  has_many :memberships, :class_name => 'AccountAdministrator'
  has_many :accounts, :through => :memberships

  accepts_nested_attributes_for :memberships

  attr_accessible :memberships_attributes
end

$ a = Administrator.new(:name => "test", :email => "test2@mail.ru", :password => "123456", :memberships_attributes => {"2"=>{"membership_role"=>"SuperAdmin", "enabled"=>"0"}, "1"=>{"membership_role"=>"Guest", "enabled"=>"1"}})