Lowell R. Beck, president of the National Association of Independent Insurers, stood before an audience of about 100 people in a hotel meeting room Wednesday and warned them of what they were about to see. Noting that he was the parent of three teen-agers, he said: "If you're not involved with kids, you're going to be a little uncomfortable with this film, because you're going to see them doing what they do."

The NAII then showed the audience of government officials, congressional staffers, insurance executives, educators and law enforcement officials a movie called "Just Along for the Ride." The film--part of an industry response to the enormous cost in lives, injuries and therefore premiums caused by drunken drivers -- is aimed particularly at teen-agers, a group that is four times more likely to have an alcohol-related crash than any other group of drivers.

The film shows why.

It is the story of three likable youngsters, their friends, and how an ordinary day and a typical teen-agers' party degenerates into tragedy. There are no adults in the movie, except at the end. The homes are upscale, suburban, with long driveways and wooded grounds. The three main characters are shown doing chores and thinking about grades. One, Danny, talks about acing a trigonometry exam and refers to himself as "the future chief of surgery." His girlfriend, Lisa, nudges him about studying for a Spanish exam.

Most of the actors were professionals who spent a week making the film in Ohio. But the third main character, "D.J.," was played by John Mitchell, a sophomore theatre major at Northwestern University.

"It started out with a regular script," he said. "When we got ahold of it, we realized it was totally outdated in terms of language. It was really preachy. Through the whole process, we improvised off the script."

The result is a look into the social lives of teen-agers that will ring true to a teen-aged audience and any parent of teen-agers who has been paying attention.

These are not troubled or delinquent kids; they are happy, clowning, fun-loving, enamored of cars and bent on having a good time. "That's all there is," says D.J. at one point, "is having a good time. You sip some brew and have a good time."

The film shows the kids beginning to drink casually in the afternoon. Lisa is an exception. She doesn't drink much, and she cautions Danny to "lay off the stuff. If you start drinking at four, your party's going to be over at five." He resents it.

The film shows the peer pressure, the ready availability of liquor in parents' homes, the absence of the parents, and the ignorance that young people have about how much they can handle. Mike, a college student who travels with a cooler of beer in his car trunk, declares at one point: "I don't care how wasted I get. I'm in control. I drive better wasted than most people do sober."

Three girls are seen sitting in a house, sipping wine, beer and tequila and making plans for the party. The boys show up in front of the picture window, holding cans of beer, clowning around and pretending to shoot each other. The scene, Mitchell said, just happened. But the symbolism is there.

The party is a parent's nightmare: beer and liquor abound. Nearly everyone is drinking, except Lisa. When the kids leave to get pizza, she wants to drive, but Danny, telling her she makes him feel like a baby, refuses. She goes with another boy who also has been drinking, but who tells his girlfriend to drive. Both cars leave, with Danny barely in control of D.J.'s car. He weaves and then misses a stop sign. D.J. yells "There's a car coming." They crash, and suddenly the screen is filled with Lisa's bloodied face and anguished expression.

It cost the NAII $200,000 to film a painfully realistic look into the beer-laced world of many American teen-agers. The organization is going to lend the movie to schools around the country. Fittingly, it was previewed here during the week that President Reagan has designated to focus national attention on drunken driving.

This is a film written, not by adults but by young people, with their feelings, their antics, and their jargon; it is the kind of film that may make others think twice before drinking and driving.

In a society that loses about 8,000 of its youths a year from alcohol-related accidents, that is surely one of the best investments the industry has made.