An ABC News "Nightline" telephone straw poll on the invasion of Grenada drew near-record numbers of callers Friday night, jamming telephone lines to record an 8-to-1 majority in favor of the American intervention.

The result does not mean that the American people are 8 to 1 in favor of the invasion.

Telephone company officials speculate that those who call in on a straw poll are those most likely to have strong feelings on the subject. Some also believe that a few people call in votes many times.

The unscientific call-in poll registered about 502,000 votes in favor of the Grenada invasion and 63,000 votes against it.

By contrast, a Washington Post-ABC News Poll using scientific polling techniques found that Americans favored the invasion by 52 to 37 percent before President Reagan's Thursday night speech and by 65 to 27 percent after the speech.

Even though lines for the "Nightline" poll were jammed and many callers could not vote because they got busy signals, 565,000 calls is the greatest number received on a call-in poll since the 1980 debate between Reagan and President Carter.

American Telephone & Telegraph Co. officials said that, although the lines were jammed at times, the 8-to-1 ratio in favor of the invasion would not have been affected, since yes and no callers alike should get busy signals in proportion to the volume of calls. Their explanation:

In the 900-number system, there are nine regional centers that can register votes. The system as a whole can handle about 8,000 calls simultaneously. When more people than that call at the same time, the caller gets a "fast busy" signal indicating that the trunk line is busy. Thus, the system looks only at the area code and exchange, and then returns a busy signal without regard to whether the vote was a yes or a no.

One reason the lines were jammed more than usual for a call-in poll was that three unrelated national call-ins were being run at the same time, including the regular call-in on a Friday night video music and dance show that customarily draws about 100,000 calls.