Gilbert,
I think you need to use an intermediate entity class (e.g., a class GroupProduct) and map one-to-many associations between it and the Group and Product classes. Alternatively, map the join table to a collection of components.
In your case, since it appears there's no ID column for the GROUP_PRODUCT table, you'd probably want to use the component collection method. Create your GroupProduct class, which will be very simple. Then map the Group and Product to use it. Example below for Product (may have typos, didn't write out
test code)
Now to add to your set (assuming the correct constructor):
These are the two approaches outlined by Bauer and King in
Java Persistence for Hibernate. I'm not sure there's a way you can add the extra column otherwise. If anybody else on the Ranch knows, I'd love to see the answer, but I've always used the intermediate entity class myself.