models.rb

class User < ActiveRecord::Base
  has_many :groups, through: :group_users
end

class GroupUsers < ActiveRecord::Base
  belongs_to :user
  belongs_to :group
end

class Group < ActiveRecord::Base
  has_many :users, through: :group_users
end

class User < ActiveRecord::Base
  has_and_belongs_to_many :groups
  has_many :owned_groups, class_name: :Group
end

class Group < ActiveRecord::Base
  has_and_belongs_to_many :users
  belongs_to :owner, class_name: :User
end