using (SPSite spSite = new SPSite("SiteCollectionUrl"))
            {
                using (SPWeb spWeb = spSite.OpenWeb())
                {
                    spWeb.AssociatedVisitorGroup = spWeb.Groups["VisitorsGroupName"];
                    spWeb.AssociatedMemberGroup = spWeb.Groups["ContributorGroupName"];
                    spWeb.AssociatedOwnerGroup = spWeb.Groups["OwnerGroupName"];
                    spWeb.Update();
                }
            }