THE KING'S SINGERS, who are the National Symphony's guests for this week's pops program, are one of the world's most amazing musical organizations. This six-man group has taken as its repertoire just about anything that can be sung and a few things (such as Rossini overtures) that seem unsingable until you hear how they do it. Their material is drawn from a period covering eight centuries, more or less, from the Middle Ages to music composed just yesterday.

They sing beautifully with an orchestra to accompany them but no less beautifully a cappella. They make no distinction between classical music (which becomes popular in their performances) and pop music (which they raise to classic status).

Their program Friday at the Kennedy Center will range from Rossini and Faure' to Simon and Garfunkel, echoing the wide variety of music that can be heard on their 30-odd records.

A few echoes of Friday's Kennedy Center program can be heard on "A La Francaise," which was recorded with the Scottish Chamber Orchestra and includes some classics of modern French music. "This Is the King's Singers" is dedicated to popular songs such as "The Windmills of Your Mind," "Strawberry Fields Forever" and "Life on Mars," as well as "I'm a Train" and "The Slow Train," which are on Friday's program. "Watching the White Wheat" is a collection of folksongs of the British Isles and contains some of the most beautiful work this group has ever done. None of the songs on this disc are scheduled for the regular program, but with luck some of them might be sung as encores. THE KING'S SINGERS:

"A La Francaise" (EMI CDC 7 47708 2); "This Is the King's Singers" (EMI CDC 7 49118 2); "Watching the White Wheat" (EMI CDC 7 47506 2). Appearing Friday in the Kennedy Center Concert Hall.