The last thing 69-year-old Regis Cupples expected when he opened the door of his Bethesda home one November Saturday was to see more than 75 of his closest friends and relatives waiting inside to wish him happy birthday.

But even more surprising than the sea of faces was the cameraman who had come to make him the star of a television documentary.

Cupples, who heads his own investment company, is hardly a Washington celebrity. Nevertheless, his wife, Bette, 49, wanted to capture every moment of the event she had planned for months.

So she paid Video Ventures, a 2-year-old Silver Spring company, $250 to videotape the event.

"I am just so happy I did it," says Cupples. "We've already watched it four times. There were people there he didn't even know were there. I would advise anybody who's having a big party like that to do it."

Cupples is one of a growing number of people who are hiring videographers to record everything from weddings and bar mitzvahs to wills.

What makes it all possible, of course, is the hot-selling video cassette recorder. More than 7 million were sold in 1984, up from 4 million in 1983. In all, some 16.5 million VCRs have been sold since 1978 and are now in about 17 percent of American households with TVs, according to the Electronic Industries Association, which is projecting sales of more than 9 million this year.

And enterprising entrepreneurs like Dianne Truell, a 35-year-old secretary, who runs Video Ventures on the side from her Silver Spring apartment, are scurrying to cash in on the fast-growing new market.

"I'm a pioneer. Most companies have only been around for one or two years," says Truell.

Armed with equipment ranging in price from $700 home video cameras to professional broadcast-quality models that can cost as much as $30,000, a growing number of videographers are recording legal depositions and recreating accident scenes for court cases, making audition tapes of area musicians, running video dating services, and videotaping the contents of people's homes for insurance records.

One Silver Spring mental health counselor has even taped interviews with about 70 Washington-area therapists to help would-be clients in their selection. For a $120 fee, Therapist Preview clients can watch and listen to up to eight therapists to get a quick sense of their philosophy and try to determine which one they would feel most comfortable working with.

"I had gotten so many bum referrals myself when looking for therapy," says Therapist Preview founder Sharon Arkin, 45. "Had I had the kind of counseling that I now know is available, I might still be married."

The traditional method of finding a therapist is a "real crap shoot," says Arkin. So in 1981, she set out to find a better way. "I had never seen a VCR or a videotape. I didn't even have a friend who owned one. But I knew that there was a dating service that uses videotape to present people to each other.

"It seemed to make sense, that you could get a lot of information about therapists that way. You could hear them; you could observe their body language; you'd get a sense of who people are."

One 38-year-old Capitol Hill attorney, who asked not to be identified, says he turned to Therapist Preview when he and his woman friend were having problems.

The couple didn't want to ask their friends and relatives for a referral, nor "waste three or four sessions with the wrong therapist. By viewing them in advance, we could observe them discussing their philosophy and have a chance to make a more informed choice," he explains.

When Wayne and Kathy Kirkland of Lorton decided to set up Atlantic Talent Agency four years ago, they began videotaping their musicians.

"One of the biggest complaints our clients had was they would have to go to four or five different places to hear bands for their weddings. They would have to pay a cover charge, then often couldn't remember whom they had seen where. It was inconvenient," says Kathy Kirkland, 33.

Now clients can do one-stop shopping in the talent agency's Alexandria office, which is equipped with a 26-inch TV and stereo sound, along with videotapes of about 65 area groups.

"People thought it was a great idea right from the start," recalls Kirkland. "When we first started out, people wanted to hear the bands live after seeing the videotape. Now they pretty much rely on the video."

"You can take almost anything and apply it to video," says Charlene Canape, author of How to Capitalize on the Video Revolution: A Guide to New Business Enterprises (Holt, Rinehart and Winston, $16.95).

Canape, 36, was Business Week's media and advertising editor when she started getting calls a few years ago from people who wanted to know how to start a video business. "After a number of these calls, I started to think there was room for a book.

"Initially, people buy a VCR to watch movies. They they begin to look for other ways to use this. They're a target for other video services," she says, adding that watching a video of a wedding is much easier than a now old-fashioned home movie. "You don't have to set up the projector. You just pop the cartridge in."

Is the public really ready for all this videotape?

"All you have to do is look at the younger generation today. Their parents listen to records. They watch MTV. No one ever thought people would be watching hit songs one day. But to them, it doesn't seem strange. We write re'sume's. They may want to videotape them," says Canape.

Silver Spring videographer Truell says she studied video in college. "When I got out of college, I said 'What am I going to do with it?' TV broadcasting seemed like too high-pressure a job for me. I didn't think I could do it. Video was new. I thought well, I'll videotape weddings and bar mitzvahs."

Truell hasn't gotten rich from her 2-year-old venture yet. But she's hoping that 1985 will be a watershed year. "Videotape is so new, people are afraid of it."

She believes that soon VCRs will be as commonplace as televisions themselves. "VCRs are like TV was in the 1950s. At first not everybody had one. But within a few years, almost everyone did.

"Videotape will be priceless for special occasions. It will be like going into a time warp, seeing people who have died or whom you've lost touch with. You've got a bona fide historical heirloom there."

Some area portrait studios are also starting to offer video services in addition to their more traditional still photography.

Last May Michael Oberman, a 37-year-old former music columnist for The Washington Star, set up Warolin Video Productions in the offices of Warolin of Georgetown, an M Street portrait studio. He shot six weddings that first month and now records about 40 percent of the weddings photographed by Warolin.

But Oberman says he hasn't made as big an inroad into the high-priced Washington wedding scene as he would like. "Video tends to be more of a middle-class thing," says Oberman.

He says some people seem to shy away from video because they fear it will be too intrusive. But he adds that professional cameras today are so good that most weddings can be done with little or no additional lighting.

In addition to shooting weddings, at an average price of between $400 and $500, Oberman also offers $49 five-minute "videograms" for people who want to send an electronic message to a loved one.

"The last one I did was of a woman who had given birth while her husband was overseas in the service. She came in, said a few words and showed the baby. She thought it would be better than a still photo," says Oberman.

But not everyone is willing to hire a professional videographer when they can buy their own color video camera for $700 or less, or rent one for between about $25 to $100 a day.

Tullio Albertini, a 47-year-old Rockville father of four, has been the family videographer since he got his own video camera 2 1/2 years ago. It was hard to use at first, he admits , a Public Health Service dentist.

"There were a lot of buttons on the side of the camera to figure out. But once you get through that learning stage, it's not very difficult.

"I wish I had it when the kids were very young," says Albertini, who now shoots everything from family gatherings and celebrations to his 17-year-old son Brian's football games. Some of the football tapes have been sent to several colleges -- with the hope Brian will be offered a scholarship.

Albertini says he plans to turn the football tapes over to Brian when he gets married and has children of his own. Then when his kids start bragging about their athletic prowess, Brian will be able to plug in a cartridge and show them that "their dad wasn't such a bad football player."

Those who would rather hire a professional videographer than master the complexities of a video camera themselves must be very careful, warns Joe Harab, head of J.D.H. Photo & Video Productions of Chevy Chase and Laurel. "There are a lot of amateurs out there. I'm concerned with people getting burned. Someone buys a video camera for $1,000 and says he's a professional. I don't want to be associated with amateurs," says Harab, who uses an expensive broadcast-quality camera.

Harab recommends that would-be buyers check credentials and preview videotapes first before selecting a videographer. "Their wedding is usually a one-time event. People want it to be right."

If the event is recorded well, says Harab, "It's a great way for people to perpetuate their history. I love what I do. When you look at these tapes, it almost brings a person back to life."