Add group' button to /admin/config/people/accounts/form-display (or any other form), can then group fields inside fieldset/other HTML wrappers.
- Bug and workaround: https://drupal.org/project/field_group/issues/3064890
- Also, field groups sometimes have the wrong default tab - my bug report (patch now committed to dev)
Types of groups:
- Horizontal tabs
- Vertical tabs
- HTML elements
In theory can use this to add <div>s with css styles such as flexbox, like webform uses, but CSS to support that needs to be present on the page.