Win32 doesn't kill process groups--the concept doesn't really exist, though Job objects are a close analogy.

However, since (I think) 5.10, kill with a negative signal id (ala SysV), will terminate the identified process(es), and all descendant of that (those) process ids.

Whilst it hasn't always been available through documented APIs, Windows (since NT4) has always retained knowledge of the parent pid (ppid) that started any given process. If you have ProcessExplorer and use the "Show Process Tree" view, it is the ppid that is used to organise the hierarchy.

Hence, if a negative signal is used, for a given process id, it is possible to determine what (if any) processes were start by that id, and recursively kill them, along with any children they have.