Thanx Vaishali,
I need to use grouped products and not bundles.
I've checked how DB schema is designed, and I've found out that currently cart table doesn't have information about grouped product id, but just references to the list of simple products id.
Anyway, I could try to change the logic and build some more sql queries to get my necessary info in order to have a more structured cart.
Do you know when this functionality will be released?