Linq to SQL usando group By y order by count

Prueba esto:

var product = 
            from p in yourContext.Active_Details
            group p by p.PVersion into pgroup
            let count = pgroup.Count()
            orderby count
            select new { Count = count, PVersion = pgroup.Key };

SELECT count(ProductVersion), ProductVersion , ProductID , SubProductID 
FROM [do-not-delete-accounts].[dbo].[Activation_Details] 
group by ProductVersion,ProductID,SubProductID 
order by count(ProductVersion);

var query = 
            from p in yourContext.Activation_Details
            group p by new 
            { 
               ProductVersion = p.ProductVersion, 
               ProductID = p.ProductID,
               SubProductID = p.SubProductID 
            } 
            into pgroup
            let count = pgroup.Count()
            orderby count
            select new 
            { 
                Count = count, 
                ProductVersion = pgroup.Key.ProductVersion, 
                ProductID = pgroup.Key.ProductID,
                SubProductID = pgroup.Key.SubProductID  
            };