the porting guide was written back in the bad old days of PG 6.
PG >= 7.0 supports the ANSI outer join syntax:
CREATE OR REPLACE VIEW n_sessions_day_user AS
SELECT b.date_id, b.n_sessions_day,
coalesce(a.n_users, 0) AS members, b.n_users AS non_members
FROM visits_grouped b left outer join visits_grouped a
on (b.n_sessions_day = a.n_sessions_day
AND b.date_id = a.date_id
AND 1 = a.member_p)
WHERE b.member_p = 0;