THEATER PROGRAMS reveal much more than who plays whom.

As production staffs increasingly get more lineage, and listings get longer and longer, "Group Sales" or simply "Sales" are now commonly alloted a line in many theater programs. They are not to be confused with subscription plans. Broader-based "sales groups" provide particular audiences for particular performances. The heads of such departments can be immensely effective theater professionals; and in the rise of institutional theater, they have given respect-ability to an old role.

Among them. New York's "Theater Party Girls" are a breed apart from other group sales representatives of our institutionalized theaters. New York's "girls," as some producers sneeringly call them, have been publicly attacked as harridans by such as David Merrick, who much as he may have disliked them, had to court them to make advance "buys" on productions he knew would need all the help they could get.

The "girls" - of whom three were long the most powerful - tote up potential values of stars, creators, scripts and subject matter with an eye to selling productions to "theater parties" well in advance of openings. Even if the reviews prove negative, the producer has the advantage of having sold at least some batches of tickets.

Within the commercial theater's one-show-at-a-time framework, theater-party bookers developed a feared, awesome power. But as expenses rose higher, even a million-dollar advance from the "girls" evaporated swiftly on $100.000-plus weekly budgets.

With the rise of institutional theater over purely commercial theater the institutions themselves have taken over what began as an outside job a mark of how nonprofit theater adapts commercial schemes that have proved workable.

Wednesday morning as its Eisenhower Theater, the Kennedy Center's sales division headed by Charles Bright will be illustrating how much stock an institution can place on pre-sales.

Before an invited audience limited to 800 theater party representatives from this area. Richmond Norfolk, Hagerstown. Frederck, Lynchburg, Carlisle, York and Wilmington will have a sampling of future Kennedy Center productions, after which they'll lunch with the artists as the Center's guests.

Last year's first Kennedy Center effort in this field netted over $350.000 in ticket sales. Wednesday morning's attractions will include Constance Cummings of Arthur Kopit's incoming "Wings". Charles Repole and a line of dancers from "Whoopee" which is destined for the Opera house on Jan. 2 and Frances Sternhagen who will read a scene from the Kennedy Center-bound "On Golden Pond" with author Ernest Thompson.

Diane Frantoni of "A Chorus Line" will move over from the National to sing here "What I Did for Love." Stephen Simon its musical director, will talk of the coming Handel Festival and John Lanchbery the American Ballet Theater musical director, will discuss its programs.

These samplings, it's hoped, will encourage the group leaders to enthusiastically urge their organizations to buy blocks of tickets. Bought in batches, such tickets have small discounts. Some will be used to benefit an organization. Most simply will be bought for pleasurable convenience.

The Kennedy Center's offering is on the plush side. Expenses are shared with the individual producing firms.

Other institutions, such as the Folger Theater Group. Ford's and Arena Stage, operate on a far modest level. But they are just as serious about the need to fill seats - for once a performance has begun an empty seat is a loss forever.

Edith Cohen - who began at the defunct Washington Theater, then instigated and now heads Arena Stage's group sales - calls this "The Empty Seat Syndrome" and says that whenever she sees an empty theater seat she can't sleep nights.

Cohen, who began her working life as an English teacher, sees her group sales efforts as another aspect of her first profession.

Her narrow office has files and lists of which she is openly proud. "If we're to have an Irish play, a Russian play or one with specialized appeal. I have my lists of people who may be especially interested. We have discounts for senior citizens and students at $3.95 a performance. And for some performances we assign free seats for the underprivileged.

"My particular joy is young people," Cohen continues "A leftover from my teaching days. I like to see the theater-going habit start early. I soft-pedal the educational aspects because Zelda [Fichandler, Arena's co-founder] feels strongly and rightly that theater should avoid the classroom atmosphere.

"One of our major groups in CloseUp. This is the national organization which brings students to Washington for a week of seeing their government in operation. We sell about 400 seats a week at 20 percent discount to CloseUp groups who come in from all 50 states. We've made Arena a traditional with CloseUp.

There is real danger to the theater, though, if I get over-zealous. Suppose, as we did last year, we have a "Hamlet" everyone - individuals, that is - wants to see. If I've sold X number of our seats at a reduced rate. Arena literally loses money. That happened last year through my over-zealousness.

"In a good year, a well-balanced year, our group sales amount to about 10 percent of our box office income. In other words, without group sales, 10 percent of our seats would have been empty.

"There's another problem. We prefer small groups to large. Our 15 percent discount goes to groups numbering from 20 to 249. The 20 percent discount is for those of 250 and over. Selling a whole house or even hundreds of seats can change the flavor of an entire audience. The performers don't like this a bit. They dread full-house theater parties."

One star alert to the difference between full-house theater parties and group sales chunks is Carol Channing.

Channing makes the point that a heterogeneous collection of individuals creates the finest audience.

"A single scene may have a dozen points to get across, some with one person, some with others. Reactions in a live audience are so communicable that these reactions seem to flutter across a well-varied audience.

"With a full-house theater party, those points get lost. You then are playing to generally rich people who have the same backgrounds, wear the same clothes, belong to the same clubs, drive the same cars and are part of the same PTA, church or organization. Nearly 20000 people, they all might as well be a single individual. And that can be a very limited audience.

"The audience which is most receptive," Channing said, "can certainly be made up of groups - but of different groups from different places, different backgrounds. Then the chemistry, the interplay between performers and audience still operates."

Group sales are flourishing everywhere in the multifaceted theatrical profession.

The Oregon Shakespeare Festival every summer draws thousands, by bus, to Ashland, where it's possible to see six different productions in as many days Florida's state theater, Asolo, has a dynamic organization. And in England bus-loads of fans from Birmingham, York or Bristol arrive nightly for Danny LaRue's London performances.

Such groups may not always fill the house, but they do fill what otherwise might be empty seats.