How can two user instances have the same account - ruby

Question

In my rails app I want to create a shared account (later with different access levels). I want two instances of the User to have the same account.
My User Model has_many :accounts and my Account belongs_to :user
In the rails console when I do, e.g:
account = Account.first
account.user_id = [1, 2]
account.save
it returns true but when I check it again account.user_id = nil
How should I go on about it, then?
EDIT:
The Account Model:
class Account < ActiveRecord::Base
belongs_to :user
has_many :products
validates_presence_of :title
end
and the User model:
class User < ActiveRecord::Base
has_many :accounts
has_many :products, through: :accounts
accepts_nested_attributes_for :accounts
validates_presence_of :name
before_save do
self.email = email.downcase unless :guest?
end
validates :name, presence: true, length: { maximum: 50 }, unless: :guest?
end
EDIT 2:
I've updated the models relationship so the new Account model is:
class Account < ActiveRecord::Base
has_and_belongs_to_many :users
validates_presence_of :title
end
and the new User model is:
class User < ActiveRecord::Base
has_and_belongs_to_many :accounts
belongs_to :account
has_many :products, through: :accounts
accepts_nested_attributes_for :accounts
validates_presence_of :name
before_save do
self.email = email.downcase unless :guest?
end
validates :name, presence: true, length: { maximum: 50 }, unless: :guest?
end
I also created a new migration, a accounts_users table
class CreateAccountsUsersJoinTable < ActiveRecord::Migration
def change
create_join_table :users, :accounts do |t|
## Not sure if this is necessary
##t.index [:user_id, :account_id]
##t.index [:account_id, :user_id]
t.belongs_to :user
t.belongs_to :account
end
end
end
Still I can't have a account with multiple users