...
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> > 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. Read this article for details. Or watch this video to see how Product Sets work.
Variations - a group of Products will be listed as a Variational Item. Customers can purchase each option of Variational Product separately.
...
In the list of Magento Products, where ID 2356 is a Parent Product, ID 2355, ID 2354, and ID 2353 are Child Products
...
Find out how to calculate price and quantity for your Grouped Product in these articles: