Hello,
BAdI BBP_BS_GROUP_BE is only called in classic scenario. It won't be called when creating RFX from shopping carts.
You can use BAdI BBP_TRANSFER_GROUP (method GROUP_BID), which will be called during execution of report BBP_SC_TRANSFER_GROUPED.
Then you can split RFXs from carts based on your own logic.
Regards,
Ricardo