House Democrats and their families today retreated to the sumptuous Greenbrier resort for a private political therapy session aimed at finding a formula to restore the party's popularity.

The weekend meeting, organized by the House Democratic Caucus and underwritten by a number of major corporations and labor unions, was closed to reporters to allow a "freer exchange" among the 135 House Democrats, who paid $400 each to participate.

A number of corporate and union executives, who each contributed at least $5,000 to the organization that presented the conference, were invited to sit in on the sessions.

Attendance at the meeting was dominated by younger, newer lawmakers, who have been demanding a greater voice in the House and on the direction of the party. House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), who plans to retire in two years, did not attend. Organizers said he had an engagement in Florida.

Several other groups within the party have been going through the same process of self-examination.

Earlier this week, a group of elected Democrats, including some of the key participants here, formed their own leadership group outside the Democratic National Committee to exert more influence on the party's direction.

Today a procession of speakers, ranging from Chrysler Corp. Chairman Lee Iacocca to former representative Barbara Jordan (D-Tex.) to motivational psychologist Ira Weinstein, told them where the party had gone wrong and where they might look for solutions.

"At this point, we're good listeners," said Rep. Norman Y. Mineta (D-Calif.). "At this point, too, flagellation is part of the program."

"The most important thing I think I said," Jordan told reporters after her speech, "is that they must remember they're Democrats and not Republicans, that sometimes Democrats become so defensive about our programs in the past that we sound like pseudo-Republicans."

But at the same time, Jordan and other speakers told the Democrats that they must reexamine many of their basic policies and that they would do well to emulate some of the public relations and communications strategies that helped President Reagan carry 49 states over Walter F. Mondale last November, the spark that set off the party's soul-searching.

Ironically, the only session open to reporters will come Sunday, when one of those who helped develop Reagan's first-term public relations successes, David R. Gergen, speaks on the importance of a communications strategy.

The organizers played down the depth of Democrats' concern about the party's future.

"We're not soul-searching and we're not in the wilderness and we're not without ideas," Rep. Richard A. Gephardt (D-Mo.), chairman of the House Democratic Caucus, said Friday at the opening-day news conference. "We need to develop a strong consensus in the party on policy . . . and connect the policy with public perception."

But he said, "When you aren't successful, you have to analyze why not."

After today's sessions, Gephardt said, "I want people to get on fire about policy, and . . . rethinking the social contract that came out of the New Deal."

House Majority Leader James C. Wright Jr. (D-Tex.) said the conference "should give the idea the Democrats are expanding. They're far from dead."

There was general agreement that the party's most serious problem has been its failure to communicate a clear message. The problem, said Rep. John D. Dingell (D-Mich.), "is we project many different messages."

Weinstein, whose clients include Elizabeth Arden, Polaroid, Hertz and General Foods, said the Democrats must learn to "package" their "product" the way Reagan did. "Reagan was sold as a unified product brilliantly," he said. "The Reagan product was one of wellness -- 'I feel pretty good about me, don't tell me I'm sick.' "

But Jordan said more basic issues must be faced, and encouraged them to undertake a "line-by-line, page-by-page review" of federal programs and be prepared to discard those that no longer work, even if they touch important Democratic constituencies.

"I told them that some of the Democratic sacred cows may not survive such a review," she said. Jordan cited Social Security cost-of-living adjustments and federal farm programs as two that should be looked at.

Iacocca urged the party to grapple with what he called the "twin scandals of our time . . . the budget deficit and the trade deficit," lashing the Reagan administration for failing to take hold of both problems.

In reaction to Reagan's decision Friday not to ask Japan to extend auto export quotas that expire March 31, Iacocca warned that Chrysler would begin to import more Japanese cars, at a cost of American jobs. Several lawmakers said he indicated that next week Chrysler might announce plant closings. Iacocca said Reagan's decision would force Chrylser to rethink its economic strategies.

The conference, named for Rep. Gillis W. Long (D-La.), the past caucus chairman who died in January, was presented in part by the National Legislative Education Foundation (NLEF), headed by John McEvoy, a Washington lawyer and an adviser to Sen. Gary Hart (D-Colo.) in last year's presidential campaign.

McEvoy said the conference cost about $170,000 and that 60 percent was raised through private contributions.

Among corporate and union executives who took advantage of the offer to sit in on the sessions, sources said, were representatives from General Electric Co., the American Trial Lawyers Association, Chevron Corp., the United Auto Workers, General Motors Corp., Morgan-Stanley & Co. and Charls E. Walker and Associates, a powerful Washington lobbying firm.