You can create new SKUs directly in the Edit Product Group view. The new SKU will take on the same group attributes, like Title, categorization, and whatever other attributes are shared across the product group.
In the Catalog, search for and select a SKU in the existing group you wish to add to.
Click on the large gear icon at the top of the left sidebar and select Edit Product Group from the menu that appears.
Select ADD NEW PRODUCT.
Enter the SKU value for the new SKU, select ADD.
If desired, you can edit the pivot attribute value(s) for the new SKU from this view.
Be sure to click SAVE to save your changes!
To edit this new SKU you can reopen the edit product group view and click on the new SKU. This will open up a new window with SKU where you can edit the price, list the SKU, or perform any other actions you might want to do.