Add a SharePoint security group through code

SPWeb web = SPContext.Current.Web

web.SiteGroups.Add("MySecurityGroup", web.CurrentUser, web.CurrentUser, "My own custom security group");

SPGroup myGroup = web.SiteGroups["MySecurityGroup"];
SPRoleAssignment roleAssignment = new SPRoleAssignment(myGroup);
SPRoleDefinition roleDefinition = site.RoleDefinition["Full Control"];
roleAssignment.RoleDefinitionBindings.Add(roleDefinition);
web.RoleAssignments.Add(roleAssignment);