class Group < ActiveRecord::Base
        acts_as_nested_set
        has_many :memberships
        has_many :users, :through => :memberships
        belongs_to :owner, :class_name => "User"
end

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

class User < ActiveRecord::Base
        has_many :memberships
        has_many :groups, :through => :memberships

end