Untitled

Adding existing security group to a subsite
SPWeb currentSite = SPContext.Current.Web;
                currentSite.EnsureUser(groupOwner);
                currentSite.SiteGroups.Add(siteTitle1, currentSite.SiteUsers[groupOwner],
                currentSite.SiteUsers[groupOwner], siteDescription1);

var subWeb = currentSite.Webs["MySubSite"];

subWeb.EnsureUser(groupOwner);
subWeb.SiteGroups.Add(siteTitle1, currentSite.SiteUsers[groupOwner],
            subWeb.SiteUsers[groupOwner], siteDescription1);