M2E Pro lists Magento Grouped Product as a Single Item on the Channel by default. To list the Grouped Product as a Multi-Variational one, configure it under Stores > Configuration > Settings > M2E Pro > Interface&Magento Inventory > Variational Product Settings.
Product Set - a group of Products will be listed as a Set (Individual Item). Customers can purchase products only as a set.
Variations - a group of Products will be listed as a Variational Item. Customers can purchase each option of Variational Product separately.
What you need to know about basic quantity settings in Selling Policy:
Quantity – allows you to select Magento source for a product quantity that will be displayed on the Channel.
Quantity Percentage – allows you to set a percentage of Magento Product quantity that will be submitted to the Channel.
Conditional Quantity – allows you to set minimum and maximum product quantity values that will be displayed on the Channel.
Important! In case the quantity value of Parent or Child Products is equal to a decimal number, for example, 3.75, M2E Pro will submit the quantity available on the Channel with the value 3 in order to prevent oversells.
If Magento Inventory Management feature is configured, M2E Pro will operate product Salable Quantity when submitting product data to Channel. Find out more details in this video or in this article.
Using a particular Grouped Product as an example, we will figure out how to calculate its quantity available on Channels depending on different Selling Policy settings and the way the product is listed: