public void AddVariantGroup( 
   VariantGroup variantGroup 
)

class MyPage : System.Web.UI.Page
{
   private void AddVariantGrpProdRelated()
   {
       Product prod = new Product();
       prod = (Product)Session["Ecom.Backend.Product"];

       string varGrpID = HttpContext.Current.Request.QueryString["grpArr"];
       bool addRelation = false;

       Dynamicweb.eCommerce.Variants.VariantGroup variantGrp = Dynamicweb.eCommerce.Variants.VariantGroup.Create(varGrpID);
       if (!prod.VariantGroups.Contains(variantGrp, true))
       {
           addRelation = true;
           prod.AddVariantGroup(variantGrp);
       }

       if (addRelation)
       {
           Dynamicweb.eCommerce.Variants.VariantGroupProductRelation vgpr = new Dynamicweb.eCommerce.Variants.VariantGroupProductRelation(prod.ID, varGrpID);
           try
           {
               vgpr.ProductID = prod.ID;
               vgpr.VariantGroupID = varGrpID;
               vgpr.Save(prod.ID, varGrpID);
           }
           catch (Exception ex)
           {
               //Nothing
           }
       }
   }
}