What is the total membership for the metropolitan area in the city of Manchester?
CREATE TABLE "group_membership" (
  "city" text,
  "county" text,
  "region" text,
  "local_authority" text,
  "type" text,
  "population" real,
  "urban_area" real,
  "metropolitan_area" real
);
SELECT MAX("metropolitan_area") FROM "group_membership" WHERE "city"='Manchester';