UNDER THE IMPRESSION that he was joining the John Birch Society, Mr. Oswald Theodoric “Theo” Wren, the noted anti-immigration activist, discovered last night that he had mistakenly joined a similarly named club devoted to recreational flogging, whose members, oddly enough, were mostly immigrants from Guatemala and El Salvador. Mr. Wren is listed in serious but stable condition at Memorial Hospital and is expected to make a full recovery, with the exception of a slight impairment in his dignity.