The best way to study groovy script is through API documentation for SoapUI. If you dont know about methods defined in TestRunner or TestCase or any such variable then on groovy test step you should write something like the following code:
and this will give you the class name for TestRunner or TestCase. After that, open the API documentation and go to the desired class name, clicking on which you will be introduced to a whole lot of methods.
Hope you have found the link useful. Otherwise are you still looking for anything specific? Groovy Script, by defult context, log, testRunner vairables are available which you can see on the top of the script editor.