```ruby
require File.expand_path('../../../spec_helper', __FILE__)
require File.expand_path('../shared/windows', __FILE__)
require 'etc'

describe "Etc.group" do
  it_behaves_like(:etc_on_windows, :group)
end
```

Add spec for parallel group iteration
user
```ruby
require File.expand_path('../../../spec_helper', __FILE__)
require File.expand_path('../shared/windows', __FILE__)
require 'etc'

describe "Etc.group" do
  it_behaves_like(:etc_on_windows, :group)

  it "raises a RuntimeError for parallel iteration" do
    proc {
      Etc.group do | group |
        Etc.group do | group2 |
        end
      end
    }.should raise_error(RuntimeError)
  end

end
```
assistant