Once you've done this
group p by p.SomeId into pg
you no longer have access to the range variables used in the initial from. That is, you can no longer talk about p or bp, you can only talk about pg.
Now, pg is a group and so contains more than one product. All the products in a given pg group have the same SomeId (since that's what you grouped by), but I don't know if that means they all have the same BaseProductId.
To get a base product name, you have to pick a particular product in the pg group (As you are doing with SomeId and CountryCode), and then join to BaseProducts.
var result = from p in Products
group p by p.SomeId into pg
// join *after* group
join bp in BaseProducts on pg.FirstOrDefault().BaseProductId equals bp.Id
select new ProductPriceMinMax {
SomeId = pg.FirstOrDefault().SomeId,
CountryCode = pg.FirstOrDefault().CountryCode,
MinPrice = pg.Min(m => m.Price),
MaxPrice = pg.Max(m => m.Price),
BaseProductName = bp.Name // now there is a 'bp' in scope
};
That said, this looks pretty unusual and I think you should step back and consider what you are actually trying to retrieve.