A conference is a gathering of important people who, singly, can do nothing, but together can decide that nothing can be done.

Fred Allen (1894-1956) American humorist [b. John Florence Sullivan]
Letter to William McChesney Martin (25 Jan 1940)
    (Source)

The letter, to the then-President of the New York Stock Exchange, was written as an apology for a joke Allen had made about Wall Street, and was re-published in TIME magazine (4 Feb 1940).

Allen apparently used the line, and variations of it, at various times in his career. A variant more commonly quoted than the original shows up, without citation, in The Oxford Dictionary of Quotations:

Committee -- A group of men who individually can do nothing but as a group decide that nothing can be done.