You will need to make sure that all products (or variants) that are meant to have the same inventory, they also needs to share the same value in SKU field.
This online store is selling the same product: Black T-Shirt with a Cat Print and Blue T-Shirt with a Cat Print to both retail customer and wholesale customers. Since the retail and wholesale has a different pricing, they are created as different products on Shopify.
The retail and wholesale product of Black T-Shirt with a Cat Print are assigned to the same SKU value SKU-BLACK
Similarly, the retail and wholesale product of Blue T-Shirt with a Cat Print are assigned to the same SKU value SKU BLUE
In the app's sidebar menu, click Smart Connected Groups by SKU . Click Enable button if this is the first time you use this feature to create Inventory Groups.
The app will start to scan every products and variants of your online store and automatically create new Inventory Groups based on every unique SKU number. Depends on the size of your store, the process could take up to 10-15 minutes.