Job-seekers by the hundreds got together with prospective employers at a "job fair" at Tysons Corner the past two days, but the employers were doing the begging.

What took place was a variation on the theme of the rich getting richer, with the well-employed, in many cases, seeking and getting better employment.

"It's a crazy kind of a situation. We have high unemployment all around us. But we have a shortage of people with qualifications meeting the needs of employers we represent," said A. W. (Bill) Aberman, president of Minneapolis-based Business People Inc.

BPI's clients -- the Central Intelligence Agency, Planning Research Corp., and takeover-embattled Martin Marietta and Bendix Corp. among them -- are looking for highly trained engineers, scientific computer programmers, chemists and technicians. Many of the people they want already are working someplace else.

As a result, BPI arranged a kind of mating game, which took place at Tysons Corner Westpark Hotel. The unstated object of the game, which BPI calls a "technical career job fair," is to facilitate the movement of disaffected employes from one corporation to another.

Of course, as Aberman points out, there are some first-time weddings -- a bright, lucky college graduate landing a job with E-Systems Inc.'s Virginia-based Melpar Division, for example. E-Systems designs electronic instruments for military use, among other things.

"The kinds of people our employers are looking for don't like to make waves by going around answering newspaper ads or sending off resumes," Aberman said. "We attract the kind of people who are thinking about looking at another company. They want to switch incognito," he said. "We are not an employment agency."

The BPI fairs, conducted in an open-house format, accomplish their mating objective by getting many employers and job-seekers together under one roof. No name registration is required for the attendees, a policy that helps to cover their tracks in preparing to move from one company to another. There is no fee charged for attendance.

But, sometimes, things get a little sticky, Aberman said. Every now and then, a job-hunter might cross the tracks of the company where he or she already is employed.

What happens then? Several possibilities, according to Aberman.

"One, the person tries to avoid his employer's booth. Or, the person might move around his employer's booth, but the company he works for is so damned big that the personnel people manning the booth don't know him.

"We've also had some people who've gotten raises. An employer once saw one of his top engineers at a fair and said: 'Hey, I didn't know that guy was unhappy.' The engineer got a raise" and stayed with the company, Aberman said.

Typical of the recruiters yesterday was Input Output Computer Services Inc., a software systems research analysis firm in Waltham, Mass. "We accept entry-level people under certain conditions, but we're not looking for them today," said Francis J. Tierney, personnel manager at IOCS, which had $13 million in sales in fiscal 1982. "We are playing a different ballgame . . . We are looking for scientific computer programmers who have a high degree of sophistication," Tierney said. He said entry-level technicians and programmers should try to find work at larger companies, such as E-Systems, which had $572 million in net sales in 1981, up from $442.2 million in 1980.

Bill Cunius, deputy chief of the CIA's Washington recruitment office admitted wanting to lure trained workers from other employers. "We're looking for engineers and scientists" for the CIA's Directorate of Science and Technology -- "the same as anyone else," Cunius said.