models.rb

# user.rb
class User < ApplicationRecord
  has_many :groups_users, :class_name => 'GroupsUsers'
  has_many :groups, :through => :groups_users
end

# group.rb
class Group < ApplicationRecord
  has_many :groups_users, :class_name => 'GroupsUsers'
  #has_many :users, :through => :groups_users
  def users
    User.all # requires eager loading
  end
end

# groups_users.rb
class GroupsUsers < ApplicationRecord
  belongs_to :group
  belongs_to :user
end