class Account < ActiveRecord::Base
  has_many :manages
  has_many :users, :through => :manages
end

class User < ActiveRecord::Base
  has_many :manages
  has_many :accounts, :through => :manages
end

acc = usr.accounts.build
acc.save

usr.accounts

acc.users

class Manage < ActiveRecord::Base
  belongs_to :user
  belongs_to :account
end