Adding new section groups is only supported in SDK. It is quite simple in SDK and the steps are described below in detail in this blog post.
1. Create a new Embedded Component
2. Configure the Embedded Component UI to whatever you want it to look like with Section Groups, Fields, Buttons, Links, Lists, etc... (you have much more UI flexibility with the SDK than the Key User tool)
3. Add the Embedded Component as a facet (Tab) to the Standard UI using the Extensibility Explorer