Same here! I have the same bug on 3.5.3 with macOS Sierra.
Like @Cescomare says, the trick is to downscale it grabbing the top handles
Seems it’s been more than a year since it was first reported - Is it something that affects only a few, or is it that users don’t often need to resize Groups?
PS. @stephen I’ve included a document and a video-capture in my report through the software’s report an issue… menu