Code:
String collectionRole = persistentClass.getEntityName();
Collection subGroupMapping = configuration.getCollectionMapping( collectionRole );
OneToMany subGroupElements = ( OneToMany ) subGroupMapping.getElement();
String associatedEntity = subGroupElements.getReferencedEntityName();
// or :
// PersistentClass oneToManyRef = subGroupElements.getAssociatedClass();
// String associatedEntity = oneToManyRef.getEntityName();
// Class associatedClass = oneToManyRef.getMappedClass();
Beware, though, that you need to make sure the associations are fully resolved *before* making these calls.