In the board room of prestigious Los Angeles law firm, 16 of Ronald Reagan's closest friends and associates today whittled lists of candidates and decided who will be the finalists for the Reagan Cabinet jobs.

Operating quietly, at the instruction of the president-elect, these unofficial but highly influential advisers agree upon a "slate" of between three and seven names for each Cabinet post. A decision memorandum containing these names and the adviser's comments on each will be hand-carried from Los Angeles to Washington to the president-elect.

The group convened at 9 a.m. in the downtown offices of Attorney William French Smith, one of Reagan's close friends. They deliberated throughout the day, having lunch brought to them in their board room work chamber, rather than taking time out for a midday recess.

With Smith serving as chairman of the meeting, the advisers discussed strengths and weaknesses of each prospective Cabinet member whose name appeared on lengthy lists prepared for them in advance. The group then voted on each name, and the top-ranked candidates were included among the finalists, according to one source who was in the meeting. The final decision memo that Reagan will receive will contain the notations of the vote tally for each candidate. "There was a great deal of unanimity in some of the votes," said one of the advisers. "After all, we didn't have to eliminate it all the way down to one final selection for each department."

"President-elect Ronald Reagan told us he did not want one recommendation, but a slate of three to five names for each position." He explained that the group's primary mission was to aid Reagan by performing the necessary preliminary reviewing of the prospective candidates, adding, "the governor does not want to get involved in the nitty-gritty."

This advisory committee is composed of men who are millionaires and multimillionaires, most of them self-made. They are middle aged and older, most of them contemporaries of the president-elect and a few of them slightly older than he.

Among those attending were at least a couple who are considered strong bets to wind up with prominent jobs in the Reagan Cabinet. They are William Simon, the former secretary of the treasury who has strong support among this group to be given that job inthe Reagan years, and Casper Weinberger, longtime Reagan associate who was President Nixon's secretary of health, education and welfare and who has been suggested for several jobs in the new administration, among them secretary of state and director of the Office of Management and Budget.

Simon and Weinberger were asked to step outside the room when the departments for which they are under consideration were being discussed. In the case of Weinberger, this adviser noted, it meant the Californian had to spend a considerable time out of the room since his name was listed for several jobs.

Others attending the meeting were: Smith, 62, a senior partner in the Gibson, Dunn and Crutcher law firm; Alfred Bloomingdale; W. Glenn Campbell of the Hoover Institute; William Casey, who was Reagan's campaign manager; Theodore E. Cummings, 72, who built the Food Giant supermarket empire; Justin Dart, 73, of Dart Industries; Jacqueline Hume, a San Francisco food entrepreneur; Earle M. Jorgensen, 82, of Jorgensen Steel; Daniel Terra, who was Reagan's national finance committee chairman; Holmes P. Tuttle, 75, owner of a major Los Angeles car dealership; Charles Z. Wick, a Los Angeles investor and GOP fund-raiser; William Wilson, a California rancher and land developer; Jack Wrather, 62, whose Wrather Corp. is into oil, entertainment and real estate (his company owns the Disneyland hotel complex in Anaheim and the right to the Lone Ranger, Lassie, and Sgt. Preston of the Yukon television series), and Joseph Coors, a staunch political conservative from the Colorado brewing family.

Other members of this advisory committee who did not attend yesterday's meetings are Edwin Meese, Reagan's longtime aide who has just been appointed counselor to the president in the new White House, Michael K. Deaver, another Reagan aide who is in line for a top White House job, and Sen. Paul Laxalt (R-Nev.), who was Reagan's campaign chairman.

The advisers at the sessions were deliberating lengthy lists of prospective Cabinet members that had been compiled for Reagan by E. Pendleton James, an executive talent hunter. James conducted a detailed talent search for what was to have been a major restructuring of the Cabinet and sub-Cabinet during President Nixon's second term. However, the scandal of Watergate intervened, causing a restructuring at a level even higher than Nixon had anticipated, and much of James' efforts never came to fruition.

James also attended today's meeting. For each Cabinet position, he had compiled names according to three categories: Those who are most outstanding and well known; those who are not as well known but who could perform well in the job; and those who Reagan and his advisers might consider "long shots" for each job.

Reagan's first personnel decision, however, although confined strictly to the top White House jobs, will nevertheless have a significant impact on the way his Cabinet performs. According to a source involved in the transition, the decision to create a presidential counselor (Meese) who would be separate from the White House chief of staff but who could oversee the work of the top domestic and security advisers, was founded in part as an effort at promoting harmony within the administration.

Specifically, it was hoped that this would create the institutional framework for ending the often acrimonious relations that existed in the Nixon and Carter years between the national security adviser in the White House and the secretary of state. Under Nixon, national security adviser Henry Kissinger tended to dominate Secretary of State William Rogers, a condition which was rectified only when Kissinger finally replaced Rogers as secretary of state. Under Carter, national security adviser Zbigniew Brzezinski was in frequent discord with Secretary of State Cyrus Vance.

Now, under Reagan, the national security adviser (Reagan aide Richard Allen is believed in line for the job) will be clearly subordinate to a White House overseer, Meese, who is a policy coordinator but not a foreign policy expert and will hold a Cabinet rank equal to that of the secretary of state.

"The State versus national security adviser thing was factored into the president-elect's decision on the White House staff," said one Reagan transition adviser.