When 12-year-old Lillian Press has a party this weekend, her friends won't play the ordinary games for girls their ages. Lillian's going to show television programs - "Eight Is Enough," "Blansky's Beauties" - on her videotape recorder.

While eating ice cream and cake, they'll watch some of the 25 tapes Lillian has recorded on her Sony Betamax unit, a Christmas present from her parents, Dr. and Mrs. Harry Press of Bethesda.

The machine is very much Lillian's. Her mother says, "I think it's lovely, but it's Lil's. I haven't even tried to work it."

Attorney Stanton J. Gildenhorn and his wife, Suzan, are also among the approximately 75,000 new owners of videotape recorders (VTR). Active in state Democratic politics in Maryland, they spend a lot of time away from their Rockville home. So while away they set the unit timer and record their favorite shows - Mary Tyler Moore, Bob Newhart, old movies and news programs.

These families - and thousands of others like them all over the world - are taking advantage of one of the newest products in media technology - the home videotape recorder. Instead of adjusting their daily schedule to fit the convenience of station programmers, they tape and watch television shows at their own convenience.

Sony chairman Akio Morita vigorously predicts that the VTR will radically change television by eliminating the concept of prime time. "All time is prime time when you can shift the schedules to suit yourself," he said.

But all is not hearts and flowers with Betamax owners. Some declined to be quoted - or even interviewed - for this story for fear of being caught up in a law suit. Universal Television and Pictures and Walt Disney Productions are seeking a court order to stop the sale of the Betamax on grounds that their copyrighted material is being stolen by users of the machine.

The trail, however, probably won't begin before early 1978 and a long fight - all the way to the Supreme Court - is expected. By 1979 Sony hopes to have 1 million Betamax units in American homes. Other companies will be pushing their own versions of the VTR. So even if Sony loses the suit, enforcing a verdict in favor of Universal and Disney may be a monstrous impracticality.

For most VTR owners, the machine adds a dimension to their television set. Mr. and MRs. Thomas Stokes of merrifield, Va., set their unit timer every Saturday to record "The Muppets," and then show it to their 17-month-old son, Kevin, when he returns from the babysitter's.

"As soon as he gets home, he walks downstairs to the basement and uncovers the tape deck, and pulls a cassette out of the rack," says Stokes. "If I put on the wrong tape, he'll call out 'Dad' right away."

Stokes also has a videotape camera, which he uses to tape the antics of his wife and son. One of his biggest treats, however, is taping himself while singing. "I enjoy singing," he explains. "Popular music - like Perry Como. And I get a charge out of seeing myself on tape."

Sony, whose Betamax is the most popular - and most heavily promoted - home VTR unit, has sold more than 75,000 machines in this country since February 1976. Other companies such as Quasar are just getting into the field.

Most purchasers are in the upper-income bracket since the Betamax tape deck lists for 1,300 and the console costs $2,500.

Sony says that many celebrities have purchased the Betamax. Sammy Davis Jr, owns two. Cher Bono and Muhammad Ali also own units.

A Washington area man who wishes to remain unidentified owns a $30,000 audio-video system, which includes a Betamax, three television sets and a custom-made sereo unit.

Gildenhorn says he's building a library of old movies. Since December he has taped several films, including "Casablanca" and "The Maltese Falcon", and right now he's waiting for "Citizen Kane" to come on.

Gildenhorn frequently exchanges tapes with friends, and friends come in handY. Since most videotape cassettes run only on ehour, it's necessary to have some help in taping a program while not at home. In such cases, Gildenhorn will set his unit timer for 60 minutes and a friend will record the second or third hour on his own unit.

Building a tape library can be expensive, however. Videocassettes cost $15.95 for 60-minutes tapes and $11.95 for 30-minute tapes.

However, cost hasn't stopped many people from buying cassettes. When "Gone With the Wind" was shown on television in two parts last fall, most dealers sold out of blank cassettes.

"You can always tell when a big event is going to be shown on television," explains Fred Burke, part-owner of Professional Products Inc., in Bethesda. "We sell out of cassettes in a hurry."

The VTR hobby can be an expensive one, but Dr. Charles Greenhouse, of Potomac, for one, thinks it's "a good investment."

Besides the regular TV shows he and his wife record, he screens medical data casettes for the latest information about new techniques in his speciality (obstetrics and gynecology).

Some purchasers, like Dr. and Mrs. Frederic Stern, who have children, ages 4 to 10, aren't so sure. Just after buying the Betamax their children taped every program they could, but he says, "So fare there's very little of redeeming value. If you want to see 'Charlie's Angels' several times, it's okay, and I'm not sorry I bought it. But it's not something every family needs to buy."

After more than a year on the market, including block buster TV campaigns and notable consumer interest, have VTR's made any measurable impact on the lives of people who've bought them?

By most accounts, no.

As Stanton J. Gildenhorn puts it, "We've enjoyed it thoroughly. But it hasn't changed our lives. TV isn't that important to us."