An antidraft organization said yesterday that updated figures from around the country show that a fifth to a third of draft-age youths in some communities failed to register during the initial July 21 to Aug. 2 sign-up period.

But a Selective Service official said the community figures cited by the Committee Against Registration and Draft (CARD) are meaningless. The official said accurate information on how many youths registered with Selective Serivce will become available for the first time in about two weeks, when preliminary data from the Selective Service's computerized count are made public.

The Selective Service estimates that about 2 million 19-year-old men and 2 million 20-year-old men were theoretically subject to the registration requirements.

However, of these 4 million, about 355,000 were members of the armed forces and therefore exempt, and some additional men (the number is unknown) were out of the country, ill or otherwise unable to register during the period.

CARD said it had surveyed postmasters in early August in five major communities to see how many youths registered at Post Offices, then matched thhese numbers against Census Bureau estimates of the 19-and-20-year-olds in the area. From this, CARD director W. Barry Lynn concluded that at least one-fifth of eligible men, and perhaps as many as $500,000 nationwide, were declining to register.

Aida Bound, associate director of CARD, said yesterday that updates and surveys of other communities confirmed the earlier estimates.

As examples, she said in Eugene, Ore., about 6,500 to 6,700 men fell into

As examples, she said in Eugene, Ore., about 6,500 to 6,700 men fell in the draft age category, but less than 50 percent had registered. In the northwestern suburbs of Chicago, several jurisdictions combined had 11,000 19-and 20-year-old men residing the area, but only 8,000 registered.

In the Austin, Tex., area, one section surveyed had 9,700 eligibles but only 6,000 registrants. In Chicago, there were 63,775 eligibles but only 40,479 registrants. In Phoenix the figures were 28,000 and 22,388, in Seattle 34,000 and 23,333, in Atlanta 63,550 and 35,765, and in Boston 54,062 and 32,600.

In addition, she said, newspapers in Memphis, Richmond and Charlotte reported anywhere from one-fifth to one-third of eligible men hadn't registered.

A Selective Service official said the survey methods used by CARD were faulty and prove nothing, because youths weren't required to register at their home Post Offices and therefore were free to do it elsewhere while on vacation, traveling, away at school, and so forth. Consequently, the official said, no survey of a handful of communities could be meaningful -- only overall national figures that Selective Service would develope in its computerized count.

CARD had 54 member groups, including the American Civil Liberties Union, United Church of Christ, Americans for Democratic Action and Friends Committee on National Legislation.