In its efforts to keep the invasion of Grenada a surprise and then to present it in the most favorable light, the Reagan administration has engaged in a campaign of secrecy and news orchestration that has created conflict inside the White House and a bitter confrontation with the media.

White House spokesman Larry Speakes, described by one official as "furious" after being misled about U.S. plans to invade the Caribbean island, complained in a memo to White House chief of staff James A. Baker III, deputy chief of staff Michael K. Deaver and White House counselor Edwin Meese III that "the credibility of the Reagan administration is at stake."

It was also reported that Speakes and other administration press officials were so upset about being misled that they discussed the possibility of resigning.

Speakes confirmed sending the memo but strongly denied that he had discussed resigning.

In addition to the Pentagon refusal to let journalists accompany the invasion force or allow any journalist to travel to the island since the mission began, four western journalists were held incommunicado.

Three of them are Americans who were on the island at the time and were picked up by the U.S. invasion force and transported to the USS Guam offshore. They were being held there last night.

The four include correspondents for The Washington Post, The Miami Herald and Newsday.

Three other journalists, including a correspondent and photographer for Time magazine and a reporter from the Financial Times of London, had arrived with the four by boat Tuesday morning. They became separated from the group while on Grenada, and their whereabouts on the island are unknown. Details on Page A11.

Meanwhile, media representatives objected to the Pentagon policy of refusing to allow reporters into the war zone in Grenada. Responding to this outcry, the administration decided last night to allow reporters onto the island.

"It's our view that this should be covered by the press from now on," Baker said.

Officials said that access to Grenada was supported in private meetings Tuesday by White House communications director David R. Gergen and then by Baker and Deaver, but that the Joint Chiefs of Staff resisted until it was clear that the invasion was a success.

The formal explanation given by both the White House and Gen. John W. Vessey Jr., chairman of the joint chiefs, was that reporters were barred for their safety. However, officials acknowledged that no such action had been taken in Vietnam, El Salvador or Lebanon, where the danger was greater.

The conflict between the administration and the news media developed on two levels yesterday.

One was the clamor from the media to enter Grenada to obtain more than the official view of the invasion.

The other was a conflict between White House reporters and Speakes over his comment on the eve of the invasion. He responded to a CBS query about whether Marines had landed on Grenada by saying it was "preposterous."

Despite his private memo to Baker and other top advisers, Speakes publicly defended the conduct of the administration in throwing a blanket of secrecy around the invasion.

"The policy of the White House is to tell the truth," he said. He admitted, however, that his answer to the invasion question could have caused confusion.

Neither Speakes nor Gergen was informed about the invasion until 6 a.m. Tuesday, when the invasion was underway.

The guidance for his remark "preposterous," Speakes said, came from Bob Sims, press officer for the National Security Council, who had obtained it from John Poindexter, the deputy national security adviser.

High administration officials said that they did not know whether Poindexter had acted on his own or had been instructed to give the misleading guidance by national security adviser Robert C. McFarlane. Neither Poindexter nor McFarlane responded to requests for comment.

Using "preposterous" to refer to a landing on Grenada first came from Poindexter in guidance to Sims, who was responding to a query from CBS correspondent Bob Schieffer.

"No one put it to me in the future tense," Sims said. "I was never asked by reporters whether there would be an invasion."

Sims said that he was tired and overworked from responding to questions about Lebanon, and didn't make further inquiries, as he might have done under other circumstances.

Several officials said that the preoccupation of the news media with Lebanon, where more than 200 Marines were killed in a suicide bombing Sunday, contributed to the White House effort to shroud the Grenada invasion in secrecy.

"Everyone was overworked and focused on Lebanon," said one official, "including the press, the White House staff and probably even the president."

But after the invasion occurred, the administration promptly launched a largely successful campaign to persuade the American public that the invasion of Grenada was a measured response to a request from neighboring nations.

Their primary weapon was Eugenia Charles, the articulate prime minister of Dominica, who appeared with Reagan in the White House briefing room Tuesday morning when he gave his reasons for the invasion.

A White House official said with pride last night that Charles had been on all the evening network news shows Tuesday, on ABC's "Nightline", on two network talk shows the next morning and had been interviewed by USA Today and U.S. News & World Report.

In addition, the White House supplied a number of spokesmen, including Secretary of State George P. Shultz and Defense Secretary Caspar W. Weinberger, to explain the administration's position on television. "We had a whole phalanx of people out there," Gergen said. "It's part of the process, and there's nothing unusual about it, though it was intense."

Although the orchestration of the administration's position was not untypical of past efforts, this time it occurred against the background of the news blackout in Grenada, which meant that virtually the only view coming out on the invasion was the official White House version.

Administration officials at every level defended the orchestration and expressed pride, as Weinberger did at a news conference, that the invasion had been carried out in secrecy.

But some officials said that the White House may pay a long-term price for, as one of them put it, "advertising that our press officials just aren't told what is going on."

The official reason for Speakes and Gergen being kept in the dark about sensitive information is that then neither is thus put in the position of lying. In practice, it has meant that these and other officials sometimes give out inaccurate information because they obtain their guidance unofficially and are not informed by Baker or Deaver.

Speakes made this point in his memo to Baker, describing what had happened and concluding: "This comes from having too little information." It was learned that Speakes, press office foreign policy spokesman Les Janka and Sims were so frustrated by the incident that they actively discussed resigning.

Gergen, who was not involved in the Grenada guidance but was an active participant in the effort to change the policy excluding reporters from Grenada, was also reported to have considered quitting.

But there was no indication that White House officials, who have often dealt with their own communications officials on a need-to-know basis, and sometimes less than that, would change their policies.

At the early morning briefing yesterday Speakes held up several typewritten pages of questions from the previous day's briefing that he said he was trying to get answered. Later Janka said, "We don't have all the facts."

Weinberger defended the decision not to let reporters on Grenada as the decision of commanders in the field. "Their conclusion was they were not able to guarantee any kind of safety to anyone," he said. "We just didn't have the conditions under which we would be able to detach enough people to protect all of the newsmen, cameramen, gripmen and all that."