In the UI:

Click into your product group, then use the "Add a Product" link at the bottom of the Update Product Group page to add the product to the group.1. 

The product fields will appear at the bottom of the products list with empty fields.

Fill in the same fields that you did when creating the other products, including the fields visible after clicking the "Edit" link (MSRP, attributes, etc). Your product information has been fully entered once the exclamation point icon turns into a grey checkmark. Once finished, click Save.

In Bulk

Export your catalog and find the product group that you want to add the product to using the Variation Grouping ID. 

Once located, add a new line below the group to represent a new product.

Fill in all fields for the new product that have also been filled for all other products in the group, including Vendor SKU, Product Identifier, and Unit Price.

Add the same Variation Grouping ID value as the other products in the group, in order to group all the products together.

Refer to the Bulk Upload Template for information about valid field values, attributes, and other information as necessary.

NOTE: Delete any rows that are not the headers (first row) or the product group that is being updated in order to avoid all products from going back through the approvals process.

Once all information has been added, save the CSV and upload using the Bulk Upload tool. Use the Bulk Upload File Error guide to address any errors that may appear while attempting to upload.

After the file has successfully uploaded, the new product will appear on the product group's Update Product Group page and on the Products page.