When you create an attribute group, it has no entities by default. So you must populate it either manually or via import facilities.
A store admin has several ways to assign a group to attributes manually:
Editing existing attributes properties
Creating new attributes
It is also possible to group attributes in bulk using import facilities. For this purpose, you will need to specify the group field value in the product-attributes.csv or attributes.csv files you prepare for import. Read more about attributes import here.
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Related pages:
Creating Global Attributes
Creating Product Class Attributes