Please read the Article Why the inventory is not synced? and How does Synchronisation Work in Connected Inventory? before testing synchronisation to make sure Track Quantity, product Status, Same Initiate Inventory and Rescan (Smart Group Only) are all in placed before testing.
Test order is placed to testing synchronisation, this aims to make sure the new Inventory Group has been setup correctly and behave as you expected.
Back to your Shopify Admin screen. Click Orders at the sidebar. Click Create Order button.
Click Browser products button to select one of the product (or variant) from the Inventory Group you have just created. Click Add to Order.
Click Mark as paid to turn it into a real order.
Click Create order to confirm.
Open the Bulk Editor once again. The inventory quantity of every product (or variants) in the Inventory Group has been reduced (and in-sync).
If you find the inventory is not sync, read the Article Why the inventory is not synced? to check all are set in place for the variant is not synced.